This is an interesting question but I'm no expert AT ALL so I urge you to double and triple check my words.
I did a little test on my instance ( feddit.it ) and I notice that I have to search for something IN my instance. This way my instance "import" posts from other instances.
Eg. I listed your posts on your instance and I selected this one https://pawb.social/post/117202 then, on my instance, I searched for the url of your post and I got this page https://feddit.it/search/q/https%3A%2F%2Fpawb.social%2Fpost%2F117202/type/All/sort/TopAll/listing_type/All/community_id/0/creator_id/0/page/1 with your post now federated in my instance https://feddit.it/post/395453 There is no pre shared ID unique on all instances because an instance start to get contents from another instance only when a user try to reach something
Hope this made some sense and it can help you (tag me whenever you'll be able to create your userscript or if you need some help)