If you have received a voucher from a friend or loved one, it is not necessary to register as a user to make use of it.
The person purchasing the voucher for you will share it with you by sending a link. They can share this via messaging channels or through email. You, as the receiver, can click on the link and specify which email address you want the voucher sent to. After confirming, the voucher will be sent to your specified address.
You can, however, choose to add the voucher to your account if you have one and keep all your vouchers easily at hand in the app.
From here, you can enter this on the merchant website to redeem the voucher and spend it!