Docs feedback: can I use variables in the complex trigger syntax?

I'm working on a downstream trigger job to trigger a review app deployment. That downstream job is already using CI_COMMIT_REF_NAME and/or CI_COMMIT_REF_SLUG.

https://docs.gitlab.com/ee/ci/yaml/#complex-trigger-syntax

I would love to use the same variable in my trigger job, but it doesn't seem to be triggering anything (the job just sits in a pending state).

deploy:
  stage: deploy
  trigger:
    project: example
    branch: $CI_COMMIT_REF_NAME
  only:
    - branches