There are a few ways to get the repository ID for a project. The easiest way is:
- log in to your tenant
- go to your Project
- add "/slack" to the url
The "/slack" suffix will show you the members of the stack. If you then click on "workbench 'content' repository", you'll see the GUID for the repository in the URL.
Another way is to click on a document in your project and then click on "View in API". The URL that comes up will have the repository ID in it as well.