How do I transfer Apple Cash to apple credit card

See the following from How to make Apple Card payments - Apple Support

Pay with Apple Cash or a different bank account

If you choose Pay Now, you can use your Apple Cash balance to make an immediate one-time payment. If the balance of your Apple Cash account is less than the amount of your payment, the remaining balance is paid from your default bank account.

You can also choose a different bank account to pay your Apple Card balance:

1. Open the Wallet app on your iPhone and tap Apple Card.
2. Tap to pay.
3. Choose a payment amount and tap the Pay Now button.
4. Tap your bank account, then choose a different bank account.
5. Tap the Back button and authenticate with Face ID, Touch ID, or your passcode to make a payment.