How to run jobs when MR labels was changed?

I want to run job when specific label is added to MR.

I tried this code:

stages:
  - test_stage

test_job:
  stage: test_stage 
  rules:
    - if: $CI_MERGE_REQUEST_LABELS =~ "stage"
  script:
    - echo "It works!"

But after adding label "stage" to MR there is no reaction.