Hi Isabel,
Do you mean hardcoding some sort of standard URL that will appear on all descriptions? Or actually adding a hyperlink in the body of one of the fields in that area of the ISAD template?
If the latter, remember that you can use Markdown in AtoM to format hyperlinks if the Markdown setting is enabled. See:
If the former, I wonder if looking at how we inject a label and link for the finding aid into the body of a description might be a helpful reference to give you some ideas. In ISAD, if you have an uploaded or generated finding aid associated with a description, then in the Conditions of Access and Use area, we will insert a hyperlink to the finding aid, with a label that indicates if it was uploaded or generated.
The link is generated via this code:
And you can see how it is inserted into the ISAD(G) template here:
Hope this helps!