Hey there GrowthMindset,
The steps to redeem from your MacBook can be found here:
How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support
"Redeem a gift card on your Mac
- Find the 16-digit code on the back of the card. For some gift cards, you might need to peel or gently scratch off the label to see the code.
- On your Mac, open the App Store.
- Click your name or the sign-in button in the sidebar.
- Click Redeem Gift Card.
7. Click Use Camera and follow the instructions on the screen. If you have trouble redeeming the card, enter your code manually.
Can't redeem your gift card?"
It can be redeemed even with an active subscription. The next time you are billed, the credit from code will be used first and any remaining charge will be charged to the payment method on file.
If this is a code not provided by Apple, you'll need to reach out to the seller for instructions on how to redeem it.
Kind regards.