NPM - Custom Properties:  Link to external Website

I have done this, and then added the variable to our email alerts so that its a clickable link.

For instance, I added SupportLink as a custom node property.  Then I created a document on our Sharepoint site that explains what procedure to follow when the node goes down.  Then I added the URL of that document to the SupportLink field for that node.  In my alerts, I added the following to the message body:

Support actions ${Support_Link}

It works perfectly.