Credit, not a music card
Money added to an Apple account pays for the subscription alongside everything else the account buys — apps, storage, films, books. The recipient does not choose "music" at any point; the subscription simply draws from the balance.
That is a feature. It means the gift is not wasted if they cancel the subscription, because the balance remains usable.
It is also why a general Apple code is usually the better thing to ask for than anything narrower.
Prepaid subscription periods
Some markets have sold fixed-length prepaid subscriptions for this service. Where they exist they are clean — a set number of months, applied directly. Where they do not, account credit does the same job with more flexibility.
Tell us the country and we will say which is available.
Family sharing
Where the subscription runs on a family plan, the organiser is billed. Credit on a family member's own account will not pay the organiser's bill.
If the recipient is part of somebody else's family plan, this gift needs to go to the organiser or it needs to be something else entirely.
Region
Apple accounts are country-specific and so is the credit. The account's country cannot usually be changed while a balance is on it, which makes a mismatched code particularly annoying — the same trap as the gaming platforms.
What else the balance covers
Apps, in-app purchases, iCloud storage, films, books, and other subscriptions billed through the account. For a recipient who pays for storage, this is often the most practically useful part.
Asking for one
Country and amount, plus whether they are on somebody else's family plan. See the music guide and software gift cards.