Creates a virtual IBAN for a user or the organization.

• If customer.userId is provided, the IBAN is created for that user
• If customer.userId is omitted, the IBAN is created for the organization itself.

Additional customer fields (e.g. email, street, city) should be passed inside the customer object as required by the payment method's requiredFields (see GET /v3/config/payment-methods/iban).

Supports multipart/form-data when document file uploads are required by the payment method.