How it works
Create the topic in the Newsletter category. If you don’t tag it as #published, it won’t show up. Once you tag it as published (and only staff have permission to tag a topic as published), it will show up on Janitor’s blog page.
Also, the webhook payload is not read, and the signature is not validated. So you can manually trigger a poll by pointing your browser at that URL.
The Pull Request implementing it: https://github.com/JanitorTechnology/janitor/pull/208
Importing old entries
The “slug” in Discourse, which can be seen in the URL, becomes the
#id of the post on Janitor (for example, the slug of this topic is
newsletters-composed-in-discourse-and-published-in-janitor). If you don’t want to break old links, make sure the post title will result in a correct slug.
Also, you can click the admin wrench and change the topic timestamp. The Janitor’s blog page sorts posts by their timestamp, so old newsletter entries will line get sorted correctly.