If it doesn't show up when you search for [email protected] it means your server doesn't know about it. You'll have to be the first to subscribe to it and thus get it sent to your server.
All communities have this format:
https://instance.tld/c/communityname
You should be able to manually type in beehaw.org/c/humor and it should pop up. The pull can take longer than a regular search, try refreshing if nothing happens.
If STILL nothing happens, visit beehaw and browse their instance list. Is this one blocked? If it is, you're out of luck because it won't work. If it isn't, you're out of luck because I don't know what could be wrong.
on my iphone:
click the hamburger menu in the top right,
click search,
Search for “[email protected]”,
the sublemmy should be listed in the results,
click it, then click sidebar,
and the subscribe button should pop up.
If it says “sign up or log in to comment” then you got there from outside your instance and can’t subscribe from there.