RequestHandler.cs

public class RequestHandler : DefaultRequestHandler
{
    public SignInForm SignIn { get; private set; }

    public override bool GetAuthCredentials(IWebBrowser browserControl, IBrowser browser, IFrame frame, bool isProxy, string host, int port, string realm, string scheme, IAuthCallback callback)
    {
        try
        {
            using (this.SignIn = new SignInForm(host))
            {
                if (this.SignIn.ShowDialog() == DialogResult.OK)
                {
                    callback.Continue(this.SignIn.Username, this.SignIn.Password);
                    return true;
                }
            }
        }
        catch (Exception ex)
        {
            Console.Write("ERROR (GetAuthCredentials): " + ex.Message);
        }
        finally
        {
            callback.Dispose();
        }

        return false;
    }
}