Once your Lemmy web frontend upgrades to version 0.18, the following should all auto-link with no extra syntax:
- !community@server.com
- /c/community@server.com
- /m/community@server.com
- /u/username@server.com
These should then all link to the instance you're currently on.
Note: because this is implemented in the frontend and does not change the underlying data, other Fediverse clients will not show these the same unless they also implement this themselves.