Online:

Select ‘Add gift card’ under the payment in checkout.
Enter the gift card number and pin code in the pop-up screen.
Click “add” to apply the gift card towards your purchase.
In-store:
Display or print the barcode, gift card number, and PIN on your mobile device.
Present to the sales advisor at purchase to scan or key in.

https://www2.hm.com/pt_pt/customer-service/gift-card/terms—conditions.html

1.The gift card can be used in the country it was purchased for – with an H&M gift card, your loved one can purchase their favorite items at any H&M store or online store at hm.com in Portugal.
2.An H&M gift card can be used to pay for the entire purchase or part of it.
3.The amount you want will be deducted from the gift card, and the value remaining on the card will be shown on the receipt.
4.If the H&M gift card does not cover your entire purchase at hm.com, you can pay the rest with another payment method.
5.Once the H&M gift card has expired, the card cannot be used to pay for purchases, it cannot be reactivated, and the unused value is not returned.
6.H&M is not responsible for lost, stolen, or damaged H&M gift cards or H&M product credit cards after activation. Treat H&M gift cards and H&M product credit cards like cash.
7.This card is not a cheque guarantee, credit, or change card and cannot be exchanged for cash or be used towards the purchase of another H&M gift card.
8.H&M reserves the right to amend these terms and conditions without notice.