Hi @markf,
Unfortunately it’s not possible to build forks when you push into the fork, because non of the git hosting services support that on an API / webhook level.
What’s possible, and is definitely supported today, is when someone starts a Pull Request from the fork into the main repository (which - the main repo - you registered on bitrise.io ) then bitrise.io can build the PR, even if the source is from a fork.
This way you don’t have to setup a new app for the fork, but builds will only happen when you open a PR and when you update it (push new commits to the PR’s branch).
I hope this helps, if you’d have any questions just let us know! 