string  thURI = "https://auth.mail.ru/cgi-bin/auth";
private bool auth()
        {
            Uri st = new Uri("http://www.mail.ru");       
            string reqString = "Login=&Domain=&Password=";
            byte[] requestData = Encoding.UTF8.GetBytes(reqString);
            CookieContainer cc = new CookieContainer();
            var request = (HttpWebRequest)WebRequest.Create(thURI);
            request.Proxy = null;
            request.CookieContainer = cc;
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            using (System.IO.Stream s = request.GetRequestStream())
               s.Write(requestData, 0, requestData.Length);
            using (var response = (HttpWebResponse)request.GetResponse())
                MessageBox.Show(cc.GetCookies(st)[0].ToString()); // Proverka
        
            return true;

        }

 string reqString = "Login=&Domain=&Password=";

string reqString = string.Format("Login={0}&Domain={1}&Password={2}",textbox1.text, textbox2.text, textbox3.text);