The offer was initially available in the U.S. only and set to expire December 20, but the bonus is now available through December 24 in the U.S., Australia, Canada, Germany, Japan, and the United Kingdom, according to Thrifter.
The bonus can be applied once on up to $200 in Apple ID funds, which can be used towards purchases on the iTunes Store, Apple Books Store, App Store, a recurring iCloud storage subscription, and so forth.
To add funds directly to an Apple ID, go to Settings > Your Name > iTunes & App Store and tap your Apple ID > View Apple ID. Sign in if necessary, tap "Add Funds to Apple ID," tap the amount that you want to add, and confirm your selection. There's also a shortcut available at the bottom of the App Store.
Or, in iTunes on a Mac or PC, click on Account > View My Account… in the menu bar and then click on the Add Funds to Apple ID link. Adding funds requires a valid payment method on file and is particularly useful for prepaid credit cards.