Without knowing anything about the server receiving the request, what the endpoint expects in terms of data or authentication, this is pretty much a shot in the dark.
But according to the Microsoft Docs
you might try replacing the following line of code
using (Stream webStream = request.GetRequestStream())
using (StreamWriter requestWriter = new StreamWriter(webStream, Encoding.ASCII))
byte byteArray = Encoding.UTF8.GetBytes(DATA);
request.ContentType = "application/json";
request.ContentLength = byteArray.Length;