Trigger a build in Jenkins on pull request

[Patricia Green]

I'm trying to use webhooks to trigger a build in Jenkins when a pull request is created. I think what I need is a merge check hook in Bitbucket. I've installed the External Hooks plugin. And there is a place to add the External Merge Check Hook. 

I've added a hook in an external-hooks folder on my server and put it in as the External Merge Check Hook. But it simply doesn't seem to be triggering anything. I've tested this hook's code as a pre-commit hook and it works fine that way.

What could I possibly be doing wrong here? this seems straightforward? How can I troubleshoot this?