No. A Promo Code needs to be entered when submitting a flight search request to ensure that the applicable fares to that Promo Code are being searched for. There will not be any other places where you can enter the Promo Code except at the beginning of the flight search. If you forget to enter the Promo Code, regular fares will be shown instead.
Qatar Airways will not be responsible for any differences in fares or fare rules incurred when customers fail to enter a Promo Code.