How to easily subscribe to communities from other instances.

I’ve been using Lemmy for almost a week and I’m loving it. But some aspects are still a bit obscure to me. For instance : I have my account on lemmy.ml. I used browse.feddit.de to find some communities. I want to subscribe to reddthat.com/c/rts.

What’s the easiest way to do so ? It looks like the search function only shows communities from the instance I’m signed on.

Edit: apparently, the servers are experiencing a little slowdown. Typing !rts is the search field returned nothing at first but finally worked at some random moment after spamming ':D