Just Eat UK

Log into your Just Eat account, or register for a Just Eat Account (https://www.just-eat.co.uk/account/register). Visit Your account, go to Redeem a gift card and enter your gift card PIN. (Your gift card PIN can be found under the scratch panel of your physical card, or the 8 digit number provided in your digital gift card email.) Your account will be credited with the amount on the gift card. Alternatively, if you have a digital gift card, click or tap the 8 digit PIN in your gift card email. You can only redeem the gift card via the website, you can't redeem it with the mobile app yet.

https://www.just-eat.co.uk/info/gift-cards/faq

Please Note: Purchases of gift cards have a 14 day refund period, from date of receipt, after which they are non-refundable. Just Eat will not be able to refund gift cards which have been credited to a Just Eat account. Refunded gift cards will be cancelled and the gift card will be held to be void.

For full terms and conditions see: https://www.just-eat.co.uk/info/terms-and-conditions