using (HttpClientHandler handler = new HttpClientHandler() { AllowAutoRedirect = true, AutomaticDecompression = System.Net.DecompressionMethods.GZip | System.Net.DecompressionMethods.Deflate })
            {
                using (HttpClient client = new HttpClient(handler) { BaseAddress = new Uri(url) })
                {

                    HtmlAgilityPack.HtmlDocument HD = new HtmlAgilityPack.HtmlDocument();
                    HD.LoadHtml(client.GetStringAsync(complUrl).Result); //можно делать все асинхронно.
                   var sizerut = HD.DocumentNode.SelectSingleNode("//tr[@class='row1']");//искомый элемент
                   }
            }

HttpWebRequest request = (HttpWebRequest)WebRequest.Create("http://www.rutracker.org/forum/login.php");


request.Method = "POST";
string data = "login_username=логин&password=пароль&login=go";
byte[] byteData = Encoding.UTF8.GetBytes(data);
request.ContentLength = byteData.Length;
Stream stream = request.GetRequestStream();
stream.Write(byteData, 0, byteData.Length);
request.CookieContainer = new CookieContainer();
string response = new StreamReader(request.GetResponse().GetResponseStream()).ReadToEnd();