On the Parent App, click the three lines in the top left corner of the screen. Tap "Payments" and then tap "Payment methods". If you have not yet added a payment method, follow THESE steps to add a new payment method. Once the payment method is added, it will show up in the list. Tap the three dots to the right of the payment method you'd like to assign as the default and tap "Make default" at the bottom of the screen. You'll see a success notification pop up that your account has been updated and the payment method should say "Default" underneath it.