For instance, this one (link to a post to [email protected]): https://reddthat.com/post/20260613
Pasting it in your search bar should give you this kind of results:
You can then click on it to access the post from your instance (in this example, lemmy.zip: https://lemmy.zip/post/16918691)
I wonder if it could be something like adding a
Link: </post/1234>; rel="activitypub"header or<link rel=activitypub href=/post/1234>. Then a browser (or browser extension) could detect this canonical ActivityPub URL and offer to open it in your configured instance or app. This is basically how RSS feeds work.