Login System [C#]

            string username = USERNAMEBOXHERE.Text; // Note: This uses any raw formatted web page with user:pass format
            string password = PASSWORDBOXHERE.Text;

            string value = username + ":" + password;
            string value2 = username+ ":" + password;
            string USER = USERNAMEBOXHERE.Text;
            string PASS = PASSWORDBOXHERE.Text;

            if (USER == "" && PASS == "")
            {
                MessageBox.Show("Error, Nothing filled in.");
                this.Close();
            }
            if (PASS == "")
            {
                MessageBox.Show("Error, Password not filled in.");
                this.Close();
            }

            if (USER == "")
            {
                MessageBox.Show("Error, Username not filled in.");
                this.Close();
            }

            WebClient webClient = new WebClient();


            bool flag3 = webClient.DownloadString("YOURRAWLINKHERE").Contains(value);
            if (flag3)
            {
                this.Hide();
                MessageBox.Show("Successfully Logged In.);

                YOURREDIRECTFORMHERE m = new YOURREDIRECTFORMHERE();
                m.Show();

            }
            else
            {

                    MessageBox.Show("Incorrect credentials.");

            }