The discount code must be digit in a field which is located in the booking confirmation page, which is the one that precede the payment page.
The discount applies only to people who are registered in buscenter portal. The discount can be added to a voucher if, and only if, their sum is less than the value of the ticket to purchase.
It is possible to use only one discount code per time. If the discount code is not applicable the button apply will not be clickable and you then can proceed to purchase the ticket without using it.