As part of the authorisation flow when connecting Intesa San Paolo to TrueLayer, you, as a user may be requested to provide a Holder Code (Codice Titolare).
The Intesa San Paolo current account Holder Code is an eight-digit code obtained when signing the contract at the branch when the user opened their current account. This would have been obtained along with a pin code.
The Holder Code is used only once to set up the Online Banking account and then never asked for again so it is easy to lose or forget. It can be found in your Intesa San Paolo contract, or by contacting the bank.