Yes, this issue has been fixed and merged into 18.1 (the next lemmy version).
-
You can see the lemmy-UI github issue here.
-
You can see the fix/merge here.
And as a temporary fix (until 18.1 releases) - if you click the “create post” button, then click the “back” button, the subscribe button should magically appear.
This User Script called Fediverse Redirector auto-redirects all Community, Post, and User pages to your home instance. It works well.
Just click install on that page - it will be added to TamperMonkey (or similar).
Then click settings (under the script) and enter your home lemmy instance.
And to directly answer your question: the raw code is visible in that repo, so you could explore how the
post
redirect query was constructed.