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)
Sites can still have third-party cookies. The first party domain just needs to explicitly allow them.