How can I apply apple gift card to my Apple Music subscription?

You can't designate your Apple ID balance to pay for a particular purchase/subscription. If the subscription renewal for Apple Music is the next charge that is made against your account, the balance will be used to help pay for the subscription.

You are welcome. If you have any credit balance, it is normally charged against the balance first, and then the credit card if the balance isn't enough to cover the total cost.