You can register the coupons in-game through the following steps.

AOS 

▶ Click the gearwheel [Menu] at the top right corner of the game screen

▶ Click [Account]

▶ Click [Game ID] > [Coupon]

▶ [Enter coupon number]

AOS & iOS

▶ https://td-coupon.vespainc.com/Event/Coupon/index.html