Users cannot add topic pages to the mobile screen (when opening a topic)

Support
4

I don’t think that is possible unfortunately.

The “Add to Home Screen” functionality requires every web app to define a start_url, in which we have defined it as the root URL of your forum.

Here’s what our a2hs manifest looks like:
https://github.com/discourse/discourse/blob/229773e7a8603ef85fd25e28980be9961832a0d5/app/controllers/metadata_controller.rb#L53