Here’s the trick:

[some community](/c/some_community@server.tld)

So:

[WowThisLemmyExists](/c/wowthislemmyexists@lemmy.ca)

Gives us this link: WowThisLemmyExists

Which links to that community... but within the instance you’re currently in! So you can actually subscribe to it!

(Many thanks to the Lemmy Project Chat on Matrix!)