I want to pay for an Apple Music Membership once my free 3 month trial ends. How

As long as the Auto Renewal Option is turned on, Apple music will automatically charge the credit card assigned to the account, or use existing iTunes Store balance on the account.

Manage your Apple Music membership on your iPhone, iPad, iPod touch, Mac, or PC - Apple Support

If you turned off automatic renewal, turn it back on. Otherwise, it'll happen automatically as long as you have enough credit or balance to pay for it.

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