How can I create a form that allows only URL input?

Question

I am developing a web browser using C# and XAML.

I want to make system that when users enter texts or numbers other than the URL in a URL display form, the screen display remains the same, but only the inside of the form is automatically blanked out and ready to be reentered. I know that I need to add something to the code below, but I don't know how. Please help.

private void txtUrl_KeyUp(object sender, KeyEventArgs e)
    {
        if (e.Key == Key.Enter)
            wbSample.Navigate(txtUrl.Text);

    }