private void AddUrlToTooltip(string url)
{
    if (webBrowser1.Document != null)
    {
        HtmlElement elem = webBrowser1.Document.CreateElement("A");
        elem.SetAttribute("HREF", url);
        elem.InnerText = "Visit our Web site for more details.";

        webBrowser1.Document.Body.AppendChild(elem);
    }
}