In the Airbnb app, open Trips and then your stay. It's called confirmation code, e.g. HMAXX2P.
In your confirmation email or the Booking.com app. It's called booking number, e.g. 4123456789.
In the confirmation email from Milan Maison.
Your host uses MyMaison to manage your online check-in.
MyMaison is an Italian platform that helps hosts handle the paperwork required by law, simply and securely.
Italian law requires every host to register all guests with the State Police (Polizia di Stato) within 24 hours of arrival (Art. 109 TULPS). Hotels do the same at reception. Doing it online now means no paperwork when you arrive.