The codes come in two kinds. Some are written for new customers: a code of this kind is used in the app, on an account with no orders yet, and covers the first few orders. Others are tied to the clock: they apply during set hours, usually to delivery orders, require a minimum basket and may not apply to combo sets.
Where a code goes depends on the offer: an app code is typed into the field on the payment screen, a website code into the basket. The website may not accept an app code and the other way round, so every offer page says which one it needs. Check that the discount has been counted before you confirm the order.
Dodo Pizza announces its campaigns on its website and on social media, but it does not name a minimum basket or an end date for every code. That is why each code page states plainly where the code comes from, what we know and what we do not. The app and the website show the delivery area and opening hours, so check your address before ordering. Offers from other restaurants and shops sit in the food and delivery section.