I have wanted to link a community a couple of times, but didn't because it seems unclear how to do it actually well. I've managed to do it once, I think. Another time didn't manage.
Maybe there should be a clear and simple to use button in the format area to add a community link?
That's a dual-purpose link. You can click it and it will send you to that community but you stay on your home instance. Or you can copy the text to discover the community if your instance does not know it yet.