You can try searching the full URL https://www.kbin.social/c/boardgames kbin look ups are kind of unreliable though some magazines seem to work some don't.
Check out https://lemmy.blahaj.zone/post/61827 (specifically option 2 under find-and-subscribe) for details on how to teach lemmy about a community it doesn't known about yet.
I think kbin magazines are supposed to be subscribable from lemmy... but I've heard some rumors today that it's not working great. I think kbin.social in particular is hiding behind cloudflare captchas to try to deal with overload and I've heard those are breaking federation. All hearsay, I could be confused... but if you can't get it to work today it might be worth just trying again next week after things have settled a bit.
I realize this was 2 days ago, but just in case you haven't noticed, as of today, kbin is federating, so you can now sub to these just like you would a lemmy community.
You're doing the right thing and it should work. There is some flakiness at the moment due to server load - I had similar problems trying to sub to [email protected].
Give it a few tries and it should come up. It then took a while for the content to copy across as well.