Please format it like this: [!community@instance.ml](/c/community@instance.ml), example: !technology@beehaw.org
This link is clickable on the Lemmy web interface, and will redirect you to the community within your own instance! It doesn’t work in the Jerboa app, I don’t know about other interfaces. Note that if your instance hasn’t had any users interact with the linked community yet, you’ll get a 404 error, and you’ll have to manually search !community@instance.ml to federate them.
No, if you search on your Lemmy instance for the community, it will pull it for you. I have had trouble searching before (sometimes) !<community>@<sever> should work, but occasionally I’ve had to use the full https link. Give it a try, hth.