Please use cross posts to make posts to multiple communities. Linking to another post doesn’t work quite right yet, but crossposts do.

If your app doesn’t support crossposts yet, you can link posts together manually, simply by entering the exact same link in the URL field. Edit your first post, copy its url (the content url, not the post url), and paste it into the url field of a post to another community, and they will get detected as the same post.

Post body content doesn’t matter, you can include a post link to the original there, etc.