Access Custom properties within the desktop.json.js file

I'd like to define the base of a URL as a custom property (or something equivilent) like "http://somedomain/", then access it in the desktop.json.js file when launching the OpenWebPage command and merging it with a webpage, pdf document, etc... so it would result in fully defined URL like "http://somedomain/help.pdf"

 

 

Is this possible?

 

 

Mike