You can do that on the current paid relays easily by just typing their public key on the relay homepage and paying.

But an option to automatically invite someone to some relay without previously knowing the pubkey (because they haven't generated one yet) is a very important thing to have, nostr:npub1jlrs53pkdfjnts29kveljul2sm0actt6n8dxrrzqcersttvcuv3qdjynqn has it as a proposal and implementation, but I never read because it was too big.