The search box, at present, is more like a fetch activity box. It can only find and retrieve objects by their activitypub ID (url).

First caveat is that you have to get to the actual object. Lemmy marks this with a little fediverse icon, but it’s pretty subtle. You can’t retrieve an object from a server that doesn’t own it, even if you can see it there. That’s just the way things are. Also remember that posts and groups can be on different servers.

Second caveat is that kbin omits some information that lets azorius know which group a post is in. If you get a group not found error, you have to search for the group first, then the post.

Fetching a post doesn’t retrieve comments. I don’t believe this is possible at present. Lemmy does not include a replies collection in its Page object. You should be able to search for comments to import them, however.