The most commonly used format is !community@instance.tld (the exclamation point is important). For something like 90% of Lemmy - browser and apps - that should open the community in your home instance when clicked. Only one of the iOS apps (Memmy, Mlem? I don’t have an iPhone sadly) had issues with this format.

There are several ways to link to a community. If your instance has had someone previously search for the community, then they will all work. If this hasn’t happened (usually because your instance is small or new) then there can be problems with the “shorthand” method that begins with an !. I’ve written a full explanation in this article at the Community Search Tips community. It lists the drawbacks and advantages of each approach that I’m aware of.