Basically before the event is signed (the sig key), the event id key gets generated.

I wrote the service as a websocket server, and something even relays could directly support and get paid for each generation or use a paid credit system.

Basically before you post you pick a POW provider, send your event and a minimum POW you want, and if you have credits, it will return an event that’s ready to sign and then publish.

All Nostr clients support websockets. And all clients that can sign, already generate the event id and sign the event - using this service just deferred the event id generation to a remote service provider, before the signing.