It's easy. For a limited time, when you set your Mastercard as the default 1-Click payment method at Amazon.co.uk, you will receive a GBP5 savings on your next eligible Amazon.co.uk purchase made on or before 31 March 2019.
Restrictions apply, see T&Cs for detail.

https://www.amazon.co.uk/adlp/master...f-87c25c832e04

The promotional voucher will only be deposited in the Amazon customer account if the customer has carried out the actions for participation in the offer within the promotion period from 07/11/2018 to 20/03/2019.

The promotional voucher will be deducted automatically from the next purchase on www.amazon.co.uk at the cashier until 31/03/2019.

The promotional voucher may only be used for products sold and fulfilled by Amazon and not for Baby and Infant Formula, Kindle eBooks, Magazines, Digital Products, Gift Vouchers, or Third Party Offers, and may only be redeemed by adults. On gift wrappings, shipping costs or other costs, the action voucher will not be counted. Only one action voucher can be redeemed per person.