So, you can’t customize the conditions of the trigger, but as LeGec mentions you can customize your code to only trigger when the Pull Request is merged.
To do that, make sure your script responds to the PullRequestEvent. The conditions to test are:
- “action” is “closed“
- “merged” (inside of “pull_request”) is true
This way your script can ignore all the pings it receives when any other activity occurs on the Pull Request (including closing without merging).