Trello Powerup Variables

I am currently using the Trello Power up that let’s you attach GitLab requests to a Trello card. I am using Trello automations to send a slack message letting others know that my ticket as moved to the “In Review” column. I need a way to include the link to my GitLab merge requests.

I assume that the attached merge requests are a custom variable on Trello. However, I can’t seem to find what they would be. If anyone has some suggestions as to how to do this, or an alternative solution that will allow slack to send a link to the card and the request, that would be much appreciated.