Hi there, andromakhe_1066.
The program will ask for your Personal ID, Passcode, and Security Number or Registration Number when connecting a Santander account in QBO.
All of them information are provided by your bank. You might want to reach out to your bank's customer service support if you don't have the information handy.
Let me also share these support articles about Online Banking and how to handle the downloaded transactions:
I'll be around if you have other questions.