It’s not on the App. It is listed on the website, cruise personalizer, itinerary, excursions. But, if you booked through a TA, then you would call them to book it for you.
At least, that’s what I’ve found on the US
bookings.
Emma, if you booked the cruise through Princess/CVP, then you would call them, if you don’t see an option to book it under the cruise personalizer. TA, then you won’t see an option, call the TA.