you cannot create a private @github fork UNLESS you create a “security advisory” first
what you can do is clone or duplicate one
- https://docs.github.com/en/free-pro-team@latest/github/collaborating-with-issues-and-pull-requests/about-forks
- https://docs.github.com/en/free-pro-team@latest/desktop/contributing-and-collaborating-using-github-desktop/cloning-and-forking-repositories-from-github-desktop
- https://docs.github.com/en/free-pro-team@latest/github/creating-cloning-and-archiving-repositories/duplicating-a-repository