Oh. Darn. Sorry. I must have read too quickly. Or disjointedly.
I’m afraid you’ll need a plugin.
Here’s one that I wrote long ago. I can’t promise that it works, or is well-written (I know a good deal more about plugins now than I did then!), but it could be a start. I’d start with GitHub - discourse/discourse-plugin-skeleton: Template for Discourse plugins · GitHub and then add the bits for the webhook from plugin.rb and the serializer.