XAML data binding to a global variable?

How can I bind a TextBoxes Text to a global variable in my class in XAML? This is for Windows Phone by the way.

Here is the code:

    namespace Class
    {
    public partial class Login : PhoneApplicationPage
    {
        public static bool is_verifying = false;

        public Login()
        {
            InitializeComponent();        
        }


        private void login_button_Click(object sender, RoutedEventArgs e)
        {
            //navigate to main page
            NavigationService.Navigate(new Uri("/MainPage.xaml", UriKind.RelativeOrAbsolute));
        }

        private void show_help(object sender, EventArgs e)
        {
            is_verifying = true;
        }
      }

    }

And I want to bind a Textboxes text to "is_verifying".

Thanks.