C# WebRequest Login

 try
            {
                string user = textBox1.Text;
                string pass = textBox2.Text;

                ASCIIEncoding encoding = new ASCIIEncoding();
                string postData = "user=" + user + "&pass=" + pass + "&section=&rememberMe=false";
                byte[] data = encoding.GetBytes(postData);

                WebRequest request = WebRequest.Create("https://www.123dapp.com/Login/Index.cfm");
                request.Method = "POST";
                request.ContentType = "application/x-www-form-urlencoded";
                request.ContentLength = data.Length;

                Stream stream = request.GetRequestStream();
                stream.Write(data, 0, data.Length);
                stream.Close();

                WebResponse response = request.GetResponse();
                stream = response.GetResponseStream();

                StreamReader sr = new StreamReader(stream);
                richTextBox1.Text = (sr.ReadToEnd());

                sr.Close();
                stream.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error : " + ex.Message);
            }

https://accounts.autodesk.com/Authentication/LogOn?viewmode=iframe&ReturnUrl=%2Fauthorize%3Fviewmode%3Diframe%26logoUrl%3Dhttps%25253a%25252f%25252fwww.123dapp.com%25252fSiteFiles%25252fContent%25252f123d_weblogo.png%26logoAlt%3D123D%26customcss%3Dhttps%25253a%25252f%25252fwww.123dapp.com%25252fSiteFiles%25252fContent%25252fCSS%25252fframe.css%26overridecss%3Dtrue%26externalOptInField%3Dtrue%26realm%3Dwww.123dapp.com%26ctx%3D123dapp%26authtype%3Dsocialoradsk%26AuthKey%3D178ca927-ed9f-4acf-a394-18a113545bfd

POST /Authentication/LogOn?viewmode=iframe&ReturnUrl=%2Fauthorize%3Fviewmode%3Diframe%26logoUrl%3Dhttps%25253a%25252f%25252fwww.123dapp.com%25252fSiteFiles%25252fContent%25252f123d_weblogo.png%26logoAlt%3D123D%26customcss%3Dhttps%25253a%25252f%25252fwww.123dapp.com%25252fSiteFiles%25252fContent%25252fCSS%25252fframe.css%26overridecss%3Dtrue%26externalOptInField%3Dtrue%26realm%3Dwww.123dapp.com%26ctx%3D123dapp%26authtype%3Dsocialoradsk%26AuthKey%3D178ca927-ed9f-4acf-a394-18a113545bfd HTTP/1.1
Host: accounts.autodesk.com
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:19.0) Gecko/20100101 Firefox/19.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Referer: https://accounts.autodesk.com/LogOn?ReturnUrl=%2fauthorize%3fviewmode%3diframe%26logoUrl%3dhttps%25253a%25252f%25252fwww.123dapp.com%25252fSiteFiles%25252fContent%25252f123d_weblogo.png%26logoAlt%3d123D%26customcss%3dhttps%25253a%25252f%25252fwww.123dapp.com%25252fSiteFiles%25252fContent%25252fCSS%25252fframe.css%26overridecss%3dtrue%26externalOptInField%3dtrue%26realm%3dwww.123dapp.com%26ctx%3d123dapp%26authtype%3dsocialoradsk%26AuthKey%3d178ca927-ed9f-4acf-a394-18a113545bfd&viewmode=iframe&logoUrl=https%253a%252f%252fwww.123dapp.com%252fSiteFiles%252fContent%252f123d_weblogo.png&logoAlt=123D&customcss=https%253a%252f%252fwww.123dapp.com%252fSiteFiles%252fContent%252fCSS%252fframe.css&overridecss=true&externalOptInField=true&realm=www.123dapp.com&ctx=123dapp&authtype=socialoradsk&AuthKey=178ca927-ed9f-4acf-a394-18a113545bfd
Cookie: ASP.NET_SessionId=ogv5hb4wkeisgkqtupuv0itp; __utma=106082195.1669608604.1362929554.1362929554.1362929554.1; __utmb=106082195.1.10.1362929554; __utmc=106082195; __utmz=106082195.1362929554.1.1.utmcsr=123dapp.com|utmccn=(referral)|utmcmd=referral|utmcct=/Login/Index.cfm
Connection: keep-alive
Content-Type: application/x-www-form-urlencoded
Content-Length: 55
UserName=atest&password=btest&section=&rememberMe=false
HTTP/1.1 200 OK
Cache-Control: private
Content-Type: text/html; charset=utf-8
Server: Microsoft-IIS/7.5
X-AspNetMvc-Version: 3.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
P3P: CP="NON DSP CURa OUR IND UNI"
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 3131
Date: Sun, 10 Mar 2013 15:35:11 GMT
Connection: keep-alive