I found that if the community you want to cross post to isn't showing up in your login server's search results, go to the Communities search (filter for All), and then use the full Lemmy community syntax to search for it - "[email protected]" (make sure to include the exclamation mark).
Your server will then go out and index it and it will pop up in the search results within a few minutes. You'll then also be able to cross post to it.
