FitNesse. UserGuide.
MarkupExternalLink [add child]
A link to an external web page is created by typing the URL.

Markup Text
Displayed as:
http://www.objectmentor.com http://www.objectmentor.com

If you would like to create a link to the files section of the local FitNesse installation, you may omit the domain name.

Markup Text
Displayed as:
http://files/images/ http://files/images/