Hello,
looking at https://magit.vc/manual/forge.html#Set-your-Username-4 I see that INSTANCE-ID should be used, with example of gitlab.example.com.user. However that does not seem to work for me. When I tried to fetch topics for the repository, it asked me to provide value for gitlab.example.com.user/api/v4 key. I am not sure it this is a bug with the code or the manual, I would expect the latter.
Have a nice day and thanks for all the work on Magit and Forge.