Trial and error of course:
marker.ToolTipText = "www.myURL.com"
The Tooltip does not appear to have a Click event, so if all you want to do is display the URL you simply add the URL to the Tooltip Text.
If you want to allow customers to click on it, best is to add a Click event on the Marker which will open a Messagebox where you can add the URL for them to click the URL or go to the Web Page
marker.ToolTipText = "www.myURL.com"
The Tooltip does not appear to have a Click event, so if all you want to do is display the URL you simply add the URL to the Tooltip Text.
If you want to allow customers to click on it, best is to add a Click event on the Marker which will open a Messagebox where you can add the URL for them to click the URL or go to the Web Page