I have a webhook that currently fires on push to any branch. This triggers the webhook far too frequently. Ideally, the webhook would only fire when a pull request is merged into master. I don't see that as an option, though:

Is there a way to get additional webhook options or to customize the webhook somehow?
I don't see any way to customize the conditions of the trigger.
I would suggest to rather write code on the receiving end to trigger your action only when you detect that the push fits your conditions, e.g :
payload.ref == "refs/head/master"payload.commits[0] matches the structure of a merged pull request(<- this may require getting some extra info from the commits API)