To apply an offer code to your payment, you will first need to select the correct subscription option.
In the top right of the payment box you will see 'Enter an Offer Code', when you click on here a box will appear where you can enter and apply the offer code to your payment.
Once successfully applied, it will confirm to you how much you are paying and how long the subscription is for.
Unless stated otherwise, most offer codes are only valid for 3 days from the date of receipt. To use your offer code successfully, make sure that you've selected the correct subscription length and then type the code exactly as it appears on the email.