if you have the url for the community (usually https://<instance>/c/<community>, for example: https://beehaw.org/c/music), click on the top-right menu bar, then the magnifying glass. enter/paste the community url into the search field, click on search. it should show up as one of the results (and sometimes takes a try or two for some reason?).

the result you want will look something like this:

also, if you know the url, you can also just figure out the url for the local instance:

https://lemmy.ca/c/<community>@<instance>, for example: https://lemmy.ca/c/music@beehaw.org

hope that makes sense!