flights

Promo codes are codes that offer discounts or other things (like free shipping, for example0 for an online purchase. They are marketed to certain groups or through certain avenues or to certain areas, and may or may not apply to you. There may not be an applicable promo code at any given time, but the box still exists on the online form because they use promo codes at various other times. There are websites out there, like Coupon Cabin, that list various promo codes by company. You might find something there (try Googling "coupon codes" or "promo codes" to find the websites). If you can't find a code that pertains to you or your planned purchase, there is no harm in leaving the box blank.