In case you haven’t found out already: you need to search for it from your local instance (the search bar is located on your feed, or by your settings). Then, when that community comes up in search results (if it doesn’t, try pasting in the URL rather than search terms), click through, and join from the sidebar. Or you could apply directly, I wouldn’t worry too much about getting approved. I think that’s often just a test to make sure bots are applying, rather than “are you good enough to join.”