تسجيل الدخول الى الموقع من منظومتي winform

السلام عليكم

اريد تسجيد الدخول موقع ما عنكريق تطبيق winform

اين المشكل في ه\ا الكود من فضلكم

    public partial class Form1 : Form
    {
        string url = "/index.php?app=core&module=global&section=login";
        string username = "زكيري عبد العالي";
        string password = "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";
        string return_page = "login.aspx";
        string inputPage = "en";
        string commit = "Log+into+the+world+of+PowerPlay+Manager%21";

        private void Form1_Load(object sender, EventArgs e)
        {
            b.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(b_DocumentCompleted);
            b.Navigate(url);
        }

        private void b_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            WebBrowser b = sender as WebBrowser;
            string response = b.DocumentText;
            b.DocumentCompleted -= new WebBrowserDocumentCompletedEventHandler(b_DocumentCompleted);
            b.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(b_DocumentCompleted2);
            string postData = string.Format("username={0}&password={1}&return_page={2}&input_page={3}&submit={4}", username, password, return_page, inputPage, commit);
            ASCIIEncoding enc = new ASCIIEncoding();
            b.Navigate(url, "", enc.GetBytes(postData), "Content-Type: application/x-www-form-urlencoded\r\n");
        }


        private void b_DocumentCompleted2(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            WebBrowser b = sender as WebBrowser;
            string response = b.DocumentText;
            if (response.Contains(username))
            {
                MessageBox.Show("Login Successful");
            }
        }  


        public Form1()
        {
            InitializeComponent();
        }
    }