For new orders, the dedicated box will appear once you are at the checkout page.
For subscription orders, head to your account, click on "upcoming delivery", and add your discount code to the box at the bottom of the page.
It is important to bear in mind that one-time discount codes are automatically removed from your account once your order has been processed. However, codes such as "Student", "NHS" or "Military" will remain attached to your subscription until they reach their expiry date. 😊