since that community is on a different instance, you just need to copy & paste the community url, in this case, https://sopuli.xyz/c/sports, and paste it in the search bar at the top.

it should then give you a result with the link to the community for your instance (https://lemmy.ml/c/sports@sopuli.xyz), which you can then subscribe to using your lemmy.ml account.

hopefully that makes sense!