I was a little surprised that it couldn't resolve to site root with the standard .NET notation.
Thanks for your feedback. We will log this and will improve the widget.
You have two options from here:
1. Move the site to IIS, and run it on fixed port or
2. Use the following control - http://www.sitefinity.com/40/help/developer-manual/controls-utility-controls-resourcelinks-control.html
I hope this helps.