You can preorder if you have allergies but if only allows you to select the top 8 allergens and possibly corn. So if your allergies are more unique or potentially harder to identify you may not be able to be certain about whether or not it's allergy safe for you. Ours were far too difficult to find a safe option while preordering so we ordered when we arrived after speaking to a chef.
As I understand it though you can alter your order if you change your mind or something so it might be worth putting it in and then talking to a chef upon your arrival to make sure it's going to be safe.