Hi Toni,
Looks like it's related to this question and looks like you have figured out how to achieve what you were trying to do.
Sharing below the resolution that's in your own words from the other question:
Specifically, I wanted to send notifications to a slack channel when an item reached the "Done" column. This can be done by setting up a subscription for items in the "Done" column and using the slack channel's email address to send to.
Intention of sharing about it in this related question is to help other Community members who might be looking for solution to similar question.