You can find the IBAN of your account in the app. Select your main account on the main screen. You will then find your IBAN in the top line. It starts with "NL" and has 18 characters. Dont be surprised: As it is a Dutch IBAN, the format is slightly different from a German IBAN. That is why you have letters in the middle of the IBAN (digits 4 - 7).