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’ve been using a GreaseMonkey script to do that, I can’t find the exact one I used but there’s a bunch that do posts too, and extensions.