in order to reference a Nostr event (or a user for that matter), use: ''nostr:[eventID, or npub, or nevent, or nprofile, whatever string here]''.
Now that you know how to do things manually, you dont actually have to. You simple selected the wrong option inside primal. You picked ''copy note link'', which gives you a url you can use outside of Nostr. You should have actually selected ''copy note ID'', it automatically adds the ''nostr:'' part for you and would have resulted in this:
📝 874651d8…