using (var client = new HttpClient())
            {
                client.BaseAddress = new Uri("https://URL.com");
                var content = new FormUrlEncodedContent(new[]
                {
                new KeyValuePair<string, string>("__EVENTTARGET", ""),
                new KeyValuePair<string, string>("__EVENTARGUMENT", ""),
                new KeyValuePair<string, string>("__VIEWSTATE", "/wEPDwUJODQ1NTExNDM3ZBgBBR5fX0NvbnRyb2xzUmVxdWlyZVBvc3RCYWNrS2V5X18WAgU4Y3RsMDAkY3RsMDAkQ29udGVudFBsYWNlSG9sZGVyMSRMb2dpblN0YXR1c0NvbnRyb2wkY3RsMDEFOGN0bDAwJGN0bDAwJENvbnRlbnRQbGFjZUhvbGRlcjEkTG9naW5TdGF0dXNDb250cm9sJGN0bDAz9StvzTms9RNhJpUJEIvLE/fgmvI="),
                new KeyValuePair<string, string>("__VIEWSTATEGENERATOR", "1806D926"),
                new KeyValuePair<string, string>("__SCROLLPOSITIONX", "0"),
                new KeyValuePair<string, string>("__SCROLLPOSITIONY", "0"),
                new KeyValuePair<string, string>("__EVENTVALIDATION", "/wEdAAftMc5YJCCVXYNVVn0in7zwyuUwJ3LGjJskskkgL1MJ/Kct0ueWAA0YL7V6O1AochnANEsgx9K9r/2lixUqPdVYOVB//zNqTaUBR/8dOG4iyLATo2l1y7ugf3AadYP1/o1HSZucqxgKJG9VFVk9FehcP636nVhtOI+3n6LJDgDKOCk9wZk="),
                new KeyValuePair<string, string>("ctl00$ctl00$ContentPlaceHolder1$ContentPlaceHolder1$LoginControl$UserName", "myusername"),
                new KeyValuePair<string, string>("ctl00$ctl00$ContentPlaceHolder1$ContentPlaceHolder1$LoginControl$Password", "mypass"),
                new KeyValuePair<string, string>("ctl00$ctl00$ContentPlaceHolder1$ContentPlaceHolder1$LoginControl$LoginButton", "constvalue")
            });
                string con = content.ToString();
                var result = client.PostAsync("/Student/Grades.aspx", content).Result;

                if (result.IsSuccessStatusCode)
                {
                    Console.WriteLine(result.StatusCode.ToString());
                    string resultContent = result.Content.ReadAsStringAsync().Result;
                    Console.WriteLine(resultContent);
                }
                else
                {
                    // problems handling here
                    Console.WriteLine("Error occurred, the status code is: {0}", result.StatusCode);
                }
            }