This is probably a "duh" article to many people, and it may even be documented somewhere. However, I couldn't find it in any of my searches, so I decided to make sure it's available for others to find.
There is no explicit webhook trigger on github for merging pull requests, and the pull request trigger only mentions other non-merge pull request functions.
For awhile, I wondered why every pull request merge was triggering two webhook payloads with identical timestamps. It turns out the pull request trigger would trigger on the merge, and the push trigger would trigger at the same time.
So if you want to trigger a webhook when merging a pull request, don't worry about the pull request webhook. Just use the default push trigger and you'll be all set.
November 2, 2017