Using DataSource that contains only path for HTTP Test Reque

I think I understand how to use the DataSource in conjunction with Property Transfer and HTTP Test Request, if my DataSource contains the HTTP endpoint, for example: http://www.mysite.com/useful_stuff.html

What if my DataSource contains just the path, for example: /useful_stuff.html

How do I use a DataSource and HTTP Test Request where I specify the site and then append the path from the DataSource?