Use of iTune gift card

I’ve bought a £15 iTune gift card at the advice of Apple Support to pay iTune’s monthly charge for my iCloud storage space, which is 8 Hong Kong $, ie about 1 US$. But I don’t know how to use it for the purpose. Can anyone advise me? (Note: Previously, I registered my Hong Kong credit card in the iPhone setting for my Apple ID for paying the charge monthly. This cannot go on now because the card has expired last month. And I cannot return to Hong Kong now to activate the renewed card due to the pandemic restrictions. )