Hi @sgn , 

The syntax you provided is not supported . And the idea of getting repository_owner is not working. 

There is a GITHUB_REPOSITORY in workflow default environment variables, it always shows the main repository not the forked repo. 

When a pull_request from a forked repo changed, there will be an event happened in the upstream repo (main repo) which could trigger a workflow in main repository.  

It is not that forks themselves can run arbitrary workflows,  it is that PRs raised from forks into the upstream will get run . 

There is a previous ticket with similar question: https://github.community/t5/GitHub-Actions/Why-can-t-a-self-hosted-runner-verify-repo-owner-before-running/td-p/39261