Отправка POST запроса и получение куки - C#

POST("https://lk2.service.nalog.ru/lk/index.html", "username=000000000000&password=12345&__checkbox_rememberMe=true");
 
       private string POST(string Url, string Data)
        {
            WebRequest req = WebRequest.Create(Url);
            req.Method = "POST";
            req.Timeout = 100000;
            req.ContentType = "application/x-www-form-urlencoded";
            byte[] sentData = Encoding.GetEncoding(1251).GetBytes(Data);
            req.ContentLength = sentData.Length;
            Stream sendStream = req.GetRequestStream();
            sendStream.Write(sentData, 0, sentData.Length);
            sendStream.Close();
            WebResponse res = req.GetResponse();
            Stream ReceiveStream = res.GetResponseStream();
            StreamReader sr = new StreamReader(ReceiveStream, Encoding.UTF8);
       
            Char[] read = new Char[256];
            int count = sr.Read(read, 0, 256);
            string Out = String.Empty;
 
            while (count > 0)
            {
                String str = new String(read, 0, count);
                Out += str;
                count = sr.Read(read, 0, 256);
            }
            return Out;
        }