Opening the composer itself isn’t the hard part - Discourse already exposes a first-class entry point for that.
You can redirect users to the /new-topic endpoint with query parameters, e.g.:
https://discourse.com/new-topic?
title=Shared%20from%20iOS
&category_id=19
&body=https%3A%2F%2Fexample.com
This opens the new topic composer with fields prefilled (title, category, body) using simple query params — not post[...].
So the flow would be:
- redirect after signup / invite acceptance
- land on
/new-topic?... - composer opens immediately with the desired category and content
This is the same mechanism used by iOS sharing and other integrations, so it’s already supported and stable.
The remaining question is where best to hook the redirect (invite accept vs first login), not whether the composer itself can be opened automatically.