HttpWebRequest/Response.

http://www.codeproject.com/KB/webservices/HttpWebRequest_Response.aspx

private static void start_post()
{
//Our postvars
byte[] buffer = Encoding.ASCII.GetBytes( "test=postvar&test2=another" );
//Initialisation, we use localhost, change if appliable
HttpWebRequest WebReq = (HttpWebRequest)WebRequest.Create("http://127.0.0.1/test.php");
//Our method is post, otherwise the buffer (postvars) would be useless
WebReq.Method = "POST";
//We use form contentType, for the postvars.
WebReq.ContentType ="application/x-www-form-urlencoded";
//The length of the buffer (postvars) is used as contentlength.
WebReq.ContentLength = buffer.Length;
//We open a stream for writing the postvars
Stream PostData = WebReq.GetRequestStream();
//Now we write, and afterwards, we close. Closing is always important!
PostData.Write(buffer, 0, buffer.Length);
PostData.Close();
//Get the response handle, we have no true response yet!
HttpWebResponse WebResp = (HttpWebResponse)WebReq.GetResponse();
//Let's show some information about the response
Console.WriteLine(WebResp.StatusCode);
Console.WriteLine(WebResp.Server);

//Now, we read the response (the string), and output it.
Stream Answer = WebResp.GetResponseStream();
StreamReader _Answer = new StreamReader(Answer);
Console.WriteLine(_Answer.ReadToEnd());

//Congratulations, you just requested your first POST page, you
//can now start logging into most login forms, with your application
//Or other examples.
}