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.