If it helps anyone else... It's the GUID that looks like this Axxxxx5B-Cxxx-4B0t-8D92-489D2xxx2B6B (redacted).
In the Admin portal, go to Capacity Settings, then click on the name of the Trial capacity, or whatever other capacity name can be clicked on.
You'll then see options like Capacity Usage Report, Notifications, etc. There's a gear on the right side of the page, click that.
You'll then see the Settings tab, will show a list of people under the Service Admin, you're username should be there.
Then at the bottom of this tab it will say SKU, Region, and Capacity ID.
Copy the Capacity ID GUID, looks like this Axxxxx5B-Cxxx-4B0t-8D92-489D2xxx2B6B (redacted), and then paste that in the setup of the Capactity Metrics app.
After a few failed attempts, changing parameters, etc. I Removed the entire Workspace and then added the Metrics App anew.