You are recommended to register your ASOS gift voucher to your account straight away when you receive it. Then get ready to spend, since the credit will be added to your account immediately. Here's how to register a Gift Voucher
Log in to My Account and click on the Gift Cards & Vouchers tab.
Click on the Add Gift Card/Voucher tab, select Gift Voucher, and enter your 16-digit voucher code.
Save Gift Voucher.