Using Datalink, How is it Referenced in the Destination URL

Been trying to get datalinks understood and working. Using Grafana 7.0. In the particular use case I am applying on a bar gauge visualization.

I create the datalink in my source panel/dashboard and include ${} at the end of the url.

But what does this dashboard at this url do with the variable how does it get referenced/applied?

I did look through the documentation and search the community. Maybe I am bad at searching. I didn’t find anything.

Thanks in advance.