Accessing pull request id on pull request merge commits

I want to run some action upon merge commits to master branch, and that action requires knowing the both commit message and the pull request id.
I created a pipeline step with the following events:

  - name: PR merge action
        - master
        - push

However, the environment variable DRONE_PULL_REQUEST has no value in the pipeline. Is there any way to access the pull request id from a drone step triggered by merging a pull request?

the event condition you are using limits execution to push events - not pull events

based on the docs here - DRONE_PULL_REQUEST | Drone - that variable will always be empty for push events :frowning: