scheduled by ID number

I have a veterinary clinic, and I have several services, one of them is pet monitoring. This service can only be scheduled by clients who have previously come to the clinic with their pets, it cannot be scheduled by new clients. Is there any way to prevent new clients from scheduling? For example, that the identification number is required upon registration and that if that number does not exist, scheduling is not allowed. Is this possible or how could I do it?

Thank you