Please note, the codes you receive are Gift Cards, not discount codes.
Firstly select your products and put them in your basket.
Log in or Continue to Continue to Checkout.
When you go to the payment section click 'Add Gift Cards'
Add your gift card number(s) and PIN.
Once you have added your gift cards click save.
The balances from the total amount of your gift cards will now be deducted from your total and you can now complete your transaction. Any remaining balance will have to be paid off using a bank card.
If you have any issues you can find your local H&M customer service team here.