I'm not sure how to do this with Jerboa as I'm not currently using it, but what you do on the web page is open the search bar, then search for the community you want like this:

1. [[email protected]](/c/[email protected])
2. https://lemmy.ml/c/fossdroid

If you get no results, you will need to wait for a few minutes, then try again as it means your current server doesn't know about it until you forced it to look. Alternatively, your server may have blocked the other server, although this isn't the case as I just forced fossdroid to appear to lemmy.world.

You've gotten a good answer, but an even more.comprehensive one is https://lemmy.blahaj.zone/post/61827. Maybe having it here will help someone else with the same question.