Normally in the sidebar section of the community

Go to Communities then search and type in the full name e.g. !asklemmy@lemmy.ml

Click search and wait a moment

To know the name look at the front page for the community and its name will be under the display name (will start with a bang)

Assuming you're viewing on mobile, at the top of the page, there should be a Sidebar button. Click that and it will expand the sidebar. There should now be a Subscribe button for you to subscribe to the community.

Note that the URL should look something like <local instance url>/c/<remote channel>@<remote instance hostname>. For example, my local instance is Beehaw too and I'm trying to view AskLemmy on lemmy.ml, so it should be something like beehaw.org/c/asklemmy@lemmy.ml

This is more of a support question and belongs in !lemmy_support@lemmy.ml

Interact with it, such as a vote. The option will then appear.

You can subscribe to another community by visiting {yourinstance.com}/c/{communityname}@{otherinstance.com}

For example: https://beehaw.org/c/asklemmy@lemmy.ml