C# - Login To Web Page Using HttpWebRequest?

I am trying to login to the website below using HttpWebRequest. I am passing in the username and password using the Credentials property but keep getting back the Login page of the website. Can anyone explain what I am doing wrong.

[URL]

HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(Url);
request.ContentType = "application/x-www-form-urlencoded";
request.Credentials = new NetworkCredential(Username, Password);
request.Method = "POST";
request.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
request.Headers.Add("Accept-Language: en-us,en;q=0.5");
request.Headers.Add("Accept-Encoding: gzip,deflate");
request.Headers.Add("Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7");
request.KeepAlive = true;
request.Headers.Add("Keep-Alive: 300");
request.Referer = Url;
request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705;)";
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
using (StreamReader reader = new StreamReader(response.GetResponseStream()))
{
string tmp = reader.ReadToEnd();
}