Predefined variable to detect a branch is not getting recognized

GitLab CI/CD
1

Predefined Variable $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME is not getting recognized by the pipeline.

I have a requirement to detect a new branch merge request. In the first stage I want to print the $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME [branch_name] but it doesn’t get recognized, the ouput gives nothing in the place of its value. Only key gets printed

merge_request:
  only:
    - merge_requests
  script:
    - echo "CI_MERGE_REQUEST_SOURCE_BRANCH_NAME: $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME"
  tags:
    - runner1