How to validate Textbox to allow only website url

// Verify that this is an allowed web site.
private void webBrowser1_Navigating(object sender,
    WebBrowserNavigatingEventArgs e)
{
    if (e.Url.Host != "www.csharphelper.com")
    {
        e.Cancel = true;
        MessageBox.Show("This web site is prohibited",
            "Prohibited", MessageBoxButtons.OK, 
            MessageBoxIcon.Exclamation);
    }
}