Trying to subscribe to a community

So I'm trying to subscribe to the fountainpens community at sopilu.xyz and it seems I'm incapable of doing so.

I think the problem is that the name of their community in their own instance is not "/c/fountainpens", but "/c/fountainpens@sopuli.xyz", as you can see from their instance here, so when I look for "!fountainpens@sopuli.xyz" in our search bar, I get nothing. Even if I try typing on the browser searchbar "https://slrpnk.net/c/fountainpens@sopuli.xyz" or "https://slrpnk.net/c/fountainpens@sopuli.xyz@sopuli.xyz", I get nothing.

Am I making a mistake somewhere? And if not, is there a way to make this work, or do I have to have an account at sopuli.xyz to post in this community?