How do I add money to Apple Pay?

To Apple Pay Cash? See the following from Send, receive, and request money with Apple Pay - Apple Support

Your Apple Cash balance is automatically used when you send money. If you don't have enough balance for the payment, the remainder will be funded by one of your debit cards in Wallet. If you have no Apple Cash balance, you can use a debit card in Wallet for the entire payment amount. You can see which card is being used on the confirmation screen.

Here's how to change your payment method:

1. Tap  next to your debit card on the confirmation screen.
2. Choose a new debit card. If the payment is split, you can see two cards on the payment sheet: your Apple Cash card and the other card that you choose to fund the transaction.