how to make reservation for milan to venice return ticket purchased online on trenitalia?
Train : FrecciaBianca 9715 Departure : Milano Centrale Arrival : Venezia S. Lucia Carriage : 5; Place : 7A,8A (Mandatory reservation) ; Fare : A/R IN GIORNATA; 1 Adults; 2ª CLASSE
Where and how to make reservation for above ticket details?