Twitter HTTP Post Error 403? Any Idea?

request = (HttpWebRequest)WebRequest.Create("https://twitter.com/account/create");
request.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 6.0; de; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5 (.NET CLR 3.5.30729)";
request.ContentType = "application/x-www-form-urlencoded";
request.Referer = "https://twitter.com/signup";
request.CookieContainer = new CookieContainer();
request.CookieContainer.Add(Cookies);
request.Method = "POST";
string formString = String.Format("authenticity_token={0}&follow=&user[name]={1}&user[screen_name]={2}&user[user_password]={3}&user[email]={4}&user[send_email_newsletter]=0&recaptcha_response_field={5}&recaptcha_challenge_field={6}&commit=Create my account", HttpUtility.UrlEncode(token), HttpUtility.UrlEncode(fullname), HttpUtility.UrlEncode(username), HttpUtility.UrlEncode(password), HttpUtility.UrlEncode(email), HttpUtility.UrlEncode(solvedcaptcha), HttpUtility.UrlEncode(challenge));

byte[] postData = Encoding.ASCII.GetBytes(formString); 
Stream os = null;
try
  {
       request.ContentLength = postData.Length; 
       os = request.GetRequestStream();
       os.Write (postData, 0, postData.Length);         //Send it
   }
catch (WebException ex)
{
        MessageBox.Show ( ex.Message, "HttpPost: Request error", 
           MessageBoxButtons.OK, MessageBoxIcon.Error );
}
finally
{
         if (os != null)
         {
              os.Close();
          }
}