I need to make a POST request using .Net.
I can authenticate by GET, and so I’m trying to make a POST request on the same connection to keep my authentication.
The reason you don't stay logged in is because you don't give the HttpWebRequest a CookieContainer to keep the session id in.
See the following to StackOverflow Q&A's for your possible solution:
C# keep session id over httpwebrequest
Multiple WebRequest in same session
I hope this helped.
Any idea how the authentication is managed on the other end? IE. If it sets a cookie, then you need to make sure you account for that, see this page, and specifically this note:
Note
For security reasons, cookies are disabled by default. If you want to use cookies, use the CookieContainer property to enable cookies.