How to retrieve and set the payment method for Stripe confirmation?

I am using a custom Stripe checkout form, which works perfectly with a credit card.

For many other payment method types like sepa debit confirmation is required, so I checked the Confirm option in the createPaymentIntent sc action.
I also set the required return url and then this error remains in the console:

You cannot confirm this PaymentIntent because it’s missing a payment method. You can either update the PaymentIntent with a payment method and then confirm it again, or confirm it again directly with a payment method.

So I suppose the payment method should be set here:

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However, I can’t figure out where to retrieve the payment method that was selected by the customer and set it here. How can that be done?

Thanks for any help.