Before you had to fiddle with the markdown link manually. As of v18, you can just type out the name of a community in the !name@instan.ce format, in plain-text, and it will become a working link.
Currently if the community hasn't been discovered by your instance then you'll get a 404 error like I just did, apparently it's a known thing and you need to search for the community before accessing it, see here - https://github.com/LemmyNet/lemmy-ui/issues/1335