byte[] response;
 
WebClient webClient = new WebClient();
response = webClient.DownloadData(LOGIN_URL);
 
string viewstate = ExtractViewState(
      Encoding.ASCII.GetString(response)
   );
 
string postData = String.Format(
   "__VIEWSTATE={0}&UsernameTextBox={1}&PasswordTextBox={2}&LoginButton=Login",
   viewstate, USERNAME, PASSWORD);
 
webClient.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
response = webClient.UploadData(
        LOGIN_URL, "POST", Encoding.ASCII.GetBytes(postData)
    );
 
private string ExtractViewState(string s)
{
   string viewStateNameDelimiter = "__VIEWSTATE";
   string valueDelimiter = "value=\"";
 
   int viewStateNamePosition = s.IndexOf(viewStateNameDelimiter);     
   int viewStateValuePosition = s.IndexOf(
         valueDelimiter, viewStateNamePosition
      );
 
   int viewStateStartPosition = viewStateValuePosition + 
                                valueDelimiter.Length;
   int viewStateEndPosition = s.IndexOf("\"", viewStateStartPosition);
 
   return HttpUtility.UrlEncodeUnicode(
            s.Substring(
               viewStateStartPosition, 
               viewStateEndPosition - viewStateStartPosition
            )
         );  
}