A link to an instance is always going to be absolute so you just type the name of the instance. Most of the time the editor links it, but it still doesn’t understand some of the domain names, for example lemm.ee links but lemmy.world does not. If the editor fails to link you can prefix the name with https://lemmy.worldle lemmy.world
The link fixer bot shows how to link a community for Lemmy, but it’s different for kbin. The problem is if you use an absolute link to a community, you redirect the user away from their instance. The [email protected] format makes links relative so you stay on your own instance. Now we have the bot and it will post a fix, but better not to call the bot. Just do it right from the start.
There’s still the problem of how to do a relative link for a post. If someone knows I’d like to see that. At this point all post links are absolute and direct you away from your instance.