How to book a meeting url in advance

Book a meeting URL in advance?

You don’t need to. Just choose a unique meeting name. Rooms don’t exist until the first participant enters.

But I want to send out invites to a moderated meeting several days in advance.

Right, so create a unique meeting name and append it to https://meet.jit.si (https://meet.jit.si/youruniquemeetingname), then send to your intended participants. The uniqueness is necessary so no one else uses the same meeting name since you can’t reserve rooms in advance on meet.jit.si. Alternatively, you can create a moderated meeting using this link - https://moderated.jitsi.net/ This gives you - the Moderator - a unique link and another link for participants.

Anyway to customize the guest URL using https://moderated.jitsi.net ?

No, but I think you can use a URL shortener.