On your app, go to preferences, click dining, and you can enter your allergies. I have a son with good allergies and we always submit his allergies before we sail and then we straight to the main dining room and ask for the Maitre D as soon as we board to confirm they know about his food allergies. They usually give him a menu for the first day or have us ask for the head waiter when we arrive to eat the first night. Princess has always been great about accommodating my son.
i hope this helps.