Admittedly, the option to enter the code is a little hidden:
Go to shopping basket > Checkout > 2. payment > "Redeem voucher or credit" > Enter code : "SONY12" > Continue > 3. order overview: Check whether the discount has been deducted.

You can find the -12% discount promotion in the product description.
The product must be placed in the shopping basket.

Before payment, you can enter the discount code. The discount will then be displayed in the shopping basket and deducted.

Quote from the product page:
"12% discount on Sony

Benefit now from 12% discount on almost all items from Sony.

Simply enter the voucher code "SONY12" during the order process under "Payment". The discount will be automatically deducted from the current sales price.

Cannot be cumulated with other voucher promotions. Valid until and including 19.12.2022."