When you have made your ticket/tickets selection, you will be directed to the shopping cart where, on the right side below the purchase total, you will find a text that says:
Here you can enter your discount/promo code in the box and then click "Submit". You will then be able to see how the discount is applied to the total amount.
NOTE!
If you have received an offer to purchase your tickets at a discounted price, you have either received a discount code to enter or a link to make your purchase through with the discounted price already applied.
Unfortunately, discounts cannot be applied after the the purchase is completed.
Therefore, to take advantage of the discount, it must be entered before completing the purchase.