Some banks in Europe require your International Bank Account Number (IBAN) in order to create an Open Banking payment. If the merchant doesn’t know your IBAN, you will be asked to add it manually during the checkout so you can safely connect your bank and proceed with the payment. You can usually find your IBAN on your bank statement or by logging into your bank's online banking/app.
Example of IBAN in Germany: DE91 1000 0000 0123 4567 89
(please don't use this to make a payment)