When you want to link to a community in a post via the webinterface, once you start typing the !... notation it offers suggestions and when you select one, it'll insert the display name !... and set the url to the c/... format.
Searching for a community accepts both notations, you can even subscribe to kbin magazines via their m/... notations ( complete url )
ideally, [!community@instance.tld](/c/community@instance.tld) - that way the link works for everyone. howevery, the web interface does this automatically if you start typing !community