A brief how-to guide for those who want this in their own Matrix channels:
Follow the instructions at t2bot.io.
Install this script somewhere so it's accessible via the web. Copy the URL retrieved through
WEBHOOK_URLconstant in the script. Change
SECRETto something unguessable.
In Postmill, go to your forum -> toolbox -> webhooks, add a new webhook, set 'URL' to the URL of the script you installed, and 'secret' to the
SECRETconstant you chose in step two. Set 'event' to 'new submission'.
If everything works, a new bot should join the Matrix channel and announce new submissions next time someone posts one.