You can provide the reviews only when your appointment status marked Completed. In Thank you email there is a link clicking that will redirect you to that completed appointment details page where Review button will be available. You can then enter reviews by clicking that Review button. Review can be submitted only once for completed appointment and there is no undo. So, please double check your reviews before submitting.
After clicking the salon link as shown in above image.
Another way to give out your review is that you can download BeautyeBooking Apps (iOS or Android) and go to My Appointment and provide your service feedback under the Review tab.