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Read POST form in Angular submitted by external websites

I am developing a website with Java for the backend and Angular for frontend. There is a situation when some external websites may send data to my website using POST form. For instance,
▼ General
Request URL: https://myangularwebsite/
Request Method: POST
...
▼ Request Headers
Content-Type: application/x-www-form-urlencoded
Host: myangularwebsite
Origin: https://externalwebsite
Referer: https://externalwebsite/send.form?id=0
...
▼ Form data
ID: 0000000
TIME: 2017.06.04 11:53:58
SIGNATURE: ...geirgmGKFGJWR...
...
Now, I need to capture the form in Angular somehow, send/redirect it to the backend to validate the signature and return the answer back to Angular to proceed working with this website.
I tried posting to my website to test how it might work using Postman, but get Cannot POST /.
I know how to work with GET and URL query parameters in Angular but I think I need to process a POST request based on headers I see with Chrome DevTools 'Network' section when coming from externalwebsite to myangularwebsite.
Should I dedicate a route in the backend and expose it, for example, .../api/external in my backend and tell these websites to use this link instead of directly posting to my Angular website's homepage?
I have already read another question ( How to read form post data in Angular 2 typescript? ) which is somewhat similar but I do not think using PHP is the right way for me as the website I am developing already has an older version written in PHP.

The answer at the link you provided is correct: you cannot do it in just Javascript, you have to use some server-side code. They mention PHP as an example, but any server-side component will do, and as you have Java at your backend, let it be Java.
So, when an HTTP request comes from an external site, you have to use a server-side component to handle it. But there are some options.
If this request is made using your user browser (so it is something like a redirect, but using a POST method), then you can do the following: catch that request at your backend, output some javascript with some data to the user's browser and process that data in your Angular code. Or this could be a redirection to your main Angular entry point, it is up to you.
If this request is made by some other means (for example, this is a server-to-server request made with with curl like a notification from a Credit Card processing), with no browser involved, then you don't need to have any Javascript (Angular or whatever it could be) as they are needed for browser only. In this case you just handle the request at your server-side.
In both cases, it seems plausible to dedicate some special endpoint for handling (or landing) such externally-originated requests.

Redirecting html page

Here is my problem :
I have to perform post from java code to some page , get the data and parse it.
The problem is that only my country ip can post to this page. Requests from another ip's are rejected.
I want to find workaround.
I have added my html page on server in my country (this server accessible from all ips) . Now I am sending a get request (in open to all server) to this page from Java code.
What I want to do is to redirect my html page to post to the original page.
I tried to use redirection , but it doesn't work - from Java code I get my html page and not redirected one.
Is there any solution or my problem ?
Thanks

I tried to use redirection , but it doesn't work - from Java code I
get my html page and not redirected one.
Yes it wont work because redirection works on client side. You perform a request to your HTML page which sends back a redirect header and your Java implementation doesnt know what to do with it. Even if it did, it had to make a new request to redirected page, which means that the request to the redirected page would still be from a denied IP.
Another option is that your redirection HTML uses JavaScript window.location.assign or something like that. The point remains the same, beacause this also is a client side solution.
You have to use some kind of server side language on the host where you placed your HTML and in that server side script you have to perform a (post or get as you wish) request to only-your-country URL. This way this only-your-country URL will see that the request came from the host where the script was, not the client itself.
For example if you can use java as your server side language on the place where currently your redirection html is, then you can check out this thread: How to send simple http post request with post parameters in java

You need a reverse proxy installed on a server located in your country. If you make a request to this reverse proxy, it will make a request to the only-your-country server and when it gets a response it will forward it to you.
So the only-your-country server will receive the same request as you make to the reverse proxy, but with a source IP address changed to the IP of the reverse proxy server.

Send additional information over HTTP header

I have written a simple HTTP server using Java and want to send some additional information (puzzle parameters and a small puzzle solver program) to the client i.e. a regular browser.
Similarly, the browser is also supposed to send information (solution) back to the server.
Is there a way to do this by just transmitting this information over the HTTP headers?
Thanks a lot

the headers are usually used to add http protocol relevant information.
You should probably use either the body of the response or cookies to add the needed information.
Adding a cookie is done using the header so it kind of fits what you are asking for.
But I wonder why you need to put it in the header? it seems like what you are asking for is url parameters (client to server) and response body (server to client).

Send http headers from servlet to app in another environment

I need to send a request from a servlet to an application running in other environment (IIS) with certain information in custom headers.
I know redirecting doesn't send the headers and getServletContext().getRequestDispatcher(url) is to be used in the servlet's context only. Has anybody made this work in some other way? I was thinking in using HttpURLConnection, but would it finally redirect the browser to the targeted app ?
Thanks in advance

You can't redirect from your servlet to the external server and keep the headers, and you can't forward the incoming request to other context.
What you could do, is to use the HttpURLConnection (or other http client library such as HTTPClient) to make the request (with the custom headers you need) to the remote server and, once the operation is complete, redirect the user to the external site (customm headers are not set in this redirection).
This is a little tricky, if you elaborate your question (what do you really need to do) we can probably think about other alternatives.

AJAX? You can send some JavaScript code to the browser which sends a request to the ISS and handle (shows) its answer. It's possible to set http headers with XMLHttpRequest but it needs client side JavaScript coding and you have to find a way to send cross domain requests.

ajax problem - 200 OK in firebug but red message with no response body

I have small ajax problem related to cross domain as i see it.
On localmachine i created html example with some ajax:
in registration text field user types 'username',
on every keystroke ajax sends it to
local Tomcat, where servlet checks if that username is already used
and sends 'taken' reponse back.
No problem on localhost at all.
As soon as i type used 'username' servlet sends 'taken' response
and browser displays it.
But, when i put test html page with ajax
on remote machine (some free hosting on remote network)
that sends validation request on my localhost Tomcat,
connection is made,
in Tomcat console i see request comming,
and in firebug in Mozzila this is Console ouput:
GET http://89.216.182.25:8080/Dinamicki1/UsernameServlet?username=zik 200 OK
...but in response tab
there is not servlet response 'taken'
and message in firebug is in red color
So servers communicate well, no firewall problems, response is 200 OK
But response body is empty.
Any ideas what this red messages in firebugs are?
Thank you very much in advance.
And if anyone can recommend a some serious ajax tutorial for java
it will be highly appreciated :)

You need to use a domain-relative URL in your Ajax request:
/Dinamicki1/UsernameServlet?username=zik
Or a context-relative URL (assuming that the page is served from /Dinamicki1):
UsernameServlet?username=zik
With regard to "Ajax tutorial for Java", start here: How to use Servlets and Ajax?

You cannot use AJAX to read replies from other domains.
Your HTML must be on the same server (and same domain, port, and protocol) as the AJAX servlet.

The 200 status reported in Firebug does not indicate the validity of the cross-domain ajax call, be it successful or not.
You might want to try using a proxy method to perform the call.
E.g. JavaScript: Use a Web Proxy for Cross-Domain XMLHttpRequest Calls

I figured out how to solve it from this site:
"To allow directory browsing via Apache Tomcat change the parameter "listings" in the file conf/web.xml from false to true."
Call your page not as C:/Documents and Settings/.../page.html but as localhost:8080/your_servlet_name (page is better named index.html).
This way, you will be able to make AJAX requests to localhost:8080/your_servlet_name/something_else.

A solution that worked for me was that I had to add "www" to the url! I was using URL Rewrite, so every URL that I had (image, js, get, load, post), I needed to use full url, but it was missing "www"!

For me, It was web api(c# .NET) request and cors was not enabled.
Added header for cors on controller and it solved the problem.
[EnableCors(origins: "*", headers: "*", methods: "*")]