I believe this should fall inside the scope of a topic event trigger, but it hasn’t been implemented yet?
I have the same situation, added feature request here: Trigger webhook on topic publish
There was a similar commit recently: https://github.com/discourse/discourse/commit/66d620f7b1d73cdb3c40491517574a9d1401a79e perhaps @tgxworld knows if trigger for topic publish is on the roadmap?