When applying a promotion code on Amazon, ensure you are logged into your account.
At Checkout, below the credit card section, look for the the option "Use a gift card, voucher, or promo code." Tap on it.
A new section labeled "Add a gift card, promo code, or voucher" will appear.
Tap on the minus sign (right side of wording) to reveal the coupon box if not visible.
Enter your code, then tap the right arrow or apply so the coupon is applied it to your order.
A confirmation message will indicate successful application of the promotional code.
Feel free to proceed with completing your order.