You can't add from URL because they're not shown there in a form that the app would handle (if there even is one, I'm new to it too).

You can use the second button on the left on the bottom bar in Jerboa. It's a screen that lists your subscribed communities but you can also search for other Lemmy communities. If their home instance federates properly you should be able to find them and then you can browse and subscribe.

It won't work with communities at kbin.social because Jerboa doesn't support Kbin, only Lemmy. Not sure if there's any app with dual Kbin+Lemmy support just yet.