Branch name template

I’m working on a migration from Jenkins + Github to Gitlab CI/CD + Gitlab code repository .

When you create a Pull Request in Github, it prefixes the branch name with PR- and this cannot be edit or changed. So in Jenkins Pipeline I can easily skip certain things if its a Pull Request.

However, I cannot achieve this with Gitlab.

How can I know that this is a Merge Request ? I have looked at the environment variables but the only one which seems helpful is CI_COMMIT_BRANCH but this can be changed by the developer.