string query = "Hello World";
WebRequest myRequest = WebRequest.Create("https://myurl");
myRequest.Timeout = 60000;
myRequest.Method = "POST";
myRequest.ContentLength = query.Length;
myRequest.ContentType = "application/x-www-form-urlencoded";
StreamWriter postWriter = new StreamWriter(myRequest.GetRequestStream());
postWriter.Write(query);
postWriter.Close();
If you want to get a response back then:
WebResponse myResponse = myRequest.GetResponse();
StreamReader rdr = new StreamReader(myResponse.GetResponseStream());