Internal 500 error while http authenfication httpwebrequest C#

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C#

Parameter

I tried to sign in using httpwebrequest C# but I have faced internal 500 error..
Here is the raw data that I copied with fiddler
Please give me advice
Thank you 


Source is
   private string login_autocafe()
        {
            string hidden_value = Get_hidden_value();
            string url = "http://m.autocafe.co.kr/Home/CarmanagerLogin";
            string sendData = "__RequestVerificationToken="+ hidden_value + "&MemberName=%EA%B3%B5%EC%A3%BC%EC%8B%9D&password=1234";
            try
            {
                HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url); 
                req.Method = "POST";
                req.Accept = "text/html, application/xhtml+xml, */*";
                req.Referer = "http://m.autocafe.co.kr/Home/CarmanagerLogin";
                req.Headers.Add("Accept-Language", "ko-KR");
                req.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko";
                req.ContentType = "application/x-www-form-urlencoded";
                req.Headers.Add("Accept-Encoding", "gzip, deflate");
                req.Host = "m.autocafe.co.kr";
                req.Credentials = new NetworkCredential("%EA%B3%B5%EC%A3%BC%EC%8B%9D", "1234");
              
                req.KeepAlive = true;
                req.Headers.Add("Pragma", "no-cache");
                req.ServicePoint.Expect100Continue = false;

                req.CookieContainer = cookie; 
                req.PreAuthenticate = true;

                StreamWriter writer = new StreamWriter(req.GetRequestStream()); 
                writer.Write(sendData); 
                writer.Close();


                HttpWebResponse result = (HttpWebResponse)req.GetResponse();
                Stream respStream = result.GetResponseStream();
                string respStr = new StreamReader(respStream).ReadToEnd();
                cookie.Add(result.Cookies);

                
                if (result.StatusCode == HttpStatusCode.OK)
                {

                    return "Sucess";
                }

                else
                {
                    Console.WriteLine("Error");
                    return "ERR";
                }

            }

            catch (WebException webex)
            {
                var pageContent = new StreamReader(webex.Response.GetResponseStream()).ReadToEnd();
            
            //  setText(txt_comtiz_result, webex.Message);

            return "ERR";
                //MessageBox.Show(webex.Message);
            }


        }
        private string Get_hidden_value()
        {
            string hidden_value = "";

            WebClient client = new WebClient();
            string sourceUrl = client.DownloadString("http://m.autocafe.co.kr/Home/CarmanagerLogin");

            HtmlAgilityPack.HtmlDocument mydoc = new HtmlAgilityPack.HtmlDocument();

            mydoc.LoadHtml(sourceUrl);

            try
            {
                if (mydoc.DocumentNode != null)
                {

                    string test = mydoc.DocumentNode.SelectSingleNode("/html/body/div/input").OuterHtml;

                    hidden_value = mydoc.DocumentNode.SelectSingleNode("/html/body/div/input").Attributes["value"].Value;




                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            return hidden_value;
        }


The data I got from fiddler is below

POST http://m.autocafe.co.kr/Home/CarmanagerLogin HTTP/1.1
Accept: text/html, application/xhtml+xml, */*
Referer: http://m.autocafe.co.kr/Home/CarmanagerLogin
Accept-Language: ko-KR
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
Content-Type: application/x-www-form-urlencoded
Accept-Encoding: gzip, deflate
Host: m.autocafe.co.kr
Content-Length: 188
Connection: Keep-Alive
Pragma: no-cache
Cookie: __RequestVerificationToken=CjXgwUkuIWXIhjtph9k092MXAsQQLHtCbO-UIZihHUvJOvQXXKc8o4v5l7oClEefzz9k5VHSvJwAZmdJAAZatHL9Eq1a1h73oJrr4--vCSg1

__RequestVerificationToken=1x5V8SIxdLtR7r7znbdyYu0MF5tG5spK3DweQOVl3Jw2Zr0ADHAEqmZZog5qjcIC8tssLgfTgov6_tzBWUEMWl2UND9e6ZFsCmUHOmtLraE1&MemberName=%EA%B3%B5%EC%A3%BC%EC%8B%9D&password=1234

What I have tried:

private string Get_hidden_value()
{
string hidden_value = "";

WebClient client = new WebClient();
string sourceUrl = client.DownloadString("http://m.autocafe.co.kr/Home/CarmanagerLogin");

HtmlAgilityPack.HtmlDocument mydoc = new HtmlAgilityPack.HtmlDocument();

mydoc.LoadHtml(sourceUrl);

try
{
if (mydoc.DocumentNode != null)
{

string test = mydoc.DocumentNode.SelectSingleNode("/html/body/div/input").OuterHtml;

hidden_value = mydoc.DocumentNode.SelectSingleNode("/html/body/div/input").Attributes["value"].Value;




}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
return hidden_value;
}