Running a GitHub Actions workflow that doesn't exist yet on the default branch

Writing this mostly for future me in case I need it again. Hopefully it can help someone else too! Sometimes you need to run a GitHub Actions workflow, but it’s not on the default branch yet, because it’s still a work in progress and hasn’t been merged yet. Typically it’s just because you want to test it. If it has a pull_request trigger, no problem, it will just run automatically when you open the pull request that adds it (if you have the appropriate permissions on the repo).