How to make an annual payment for iCloud space?

As you may know, iCloud+ is a monthly subscription. Annual payments are not offered.

However, if you wanted to commit a year's worth of monthly payments you could add the value to your Apple Balance and then set that as your payment method.

Be mindful though that any other purchase made through Apple will be shared to your Apple Balance while that is set as your payment method.

You can always Check your Apple Account balance - Apple Support

Annual iCloud storage plans [Archived document = policy no longer active] - If you have an annual iCloud storage plan - Apple Support

There are now no longer any annual plans*, only monthly unless you have a very old plan that is still set to annual payment. If you do not want to add a credit card to your payment information to cover monthly payments, you can try using a gift card to make sure your Apple ID balance has enough money to make payments automatically over a long term. Note that account balance may be used to pay for anything you buy, it won't be reserved for iCloud subscription use exclusively unless you do not buy anything else. Also note though that because this is a subscription, Apple may require a valid payment form such as a credit card be present on the account even though the Apple Account balance will be used first.

*"If you're a new iCloud subscriber, you can only choose a monthly iCloud storage plan."

"This article has been archived and is no longer updated by Apple." Apple doesn't say what it means by that but it is often found on documents about a feature that is no longer actively supported.