In the Mastodon web interface, you can take the URL of the Lemmy community and paste it into the search bar. After you press Enter, the community should show up, and you can follow it.
Another way to reference the community is using @ notation. For this community, you'd reference @[email protected] in Mastodon.
I use Ivory to access my Mastodon account, and I've found that it doesn't recognize URLs from Lemmy at all. So, the @ notation works best there. Regardless, the Mastodon web interface handles it all properly.