You need to pay the subscription fee. Generally speaking, you visit their website and pay the Lightning invoice, then add the relay to your profile.
Discussion
Thank you, I didn't realise that I'd have to visit their website. Sometimes it's not clear what their website is, but for some of them like nostr.wine it looks straightforward.
I also paid for a nostrplebs.com address by the way. I've just realised that you're the developer/owner of nostrplebs, thank you so much for replying to my note!