Mirroring GitLab repository

I am wanting to mirror a GitLab repository, so that my fork will automatically stay up to date with the master. (As described on the Project Forking Workflow doc, Repository Mirroring section.)

However, all of the documentation I have read for repository mirroring leads me to believe that this feature is only available for repositories that exist outside of GitLab (e.g., remote servers such as GitHub or Bitbucket). Surely there must be a way to get around this?

I tried navigating to my forked repo > Settings > Repository > Mirroring repositories. I then entered the web address for the master repo and added “.git” to the end of it, as suggested by this post. I also tried including my username. I set the direction to “Pull” and entered my GitLab password in the “Password” field. However, I am still getting this error:

Is there any way around this? Are there any other approaches to avoiding manually pulling changes from upstream?

Thank you in advance for your help.