if(Url_txt.Text.Contains("."))
{
   //Ist eine url
   WebBrowser.Navigate(Url_txt.Text.Trim());
}
else
{
   //Ist eine Suche
   WebBrowser.Navigate("https://www.google.com/search?client=opera-gx&q="+Url_txt.Text.Trim()+"&sourceid=opera&ie=UTF-8&oe=UTF-8");
}