C# | How to Post JSON to Webserver

i have an Webspace with the domain http://www.xyz.de for example. And a program in c# where i can create user profiles. When i created a user profile, it should be that the new user profile should post to Webspace in JSON. So that it would like this: URL: http://www.xyz.de/user

JSON:

{
  "userid": 1,
  "username": "Username"
}

I have searched after results for my problem, but i get only a 404 Not Found Error or nothing and still nothing happened.