Employee should be able to add a coupon when they book
If they make a booking for customers and your coupons are enabled and applied to the services your employee want to book, they can. If your employee tries to book from the front end as a customer would do, they can add the coupon on the payment page if the coupon is enabled and applied to the selected service. So this is already implemented
It doesn’t work from the Employee panel. Employee can add appointment outside “open hours” in their calendar but not add a coupon. I don’t understand why it is not possible.