Prevent Booking without validation

[lauralibretti]

I want to put a validation on the event "createBookingStarted" which if not met, prevent processing the appointment. Something like:
...
callbacks: {
createBookingStarted: function(args) { BookingClick(args) }
} ......

function BookingClick (args) {
console.log('createBookingStarted', args)
if (ValidationNotOK){
//Stop process
return false
}
}

Thank you in advance!