Heroes War: Counterattack has an option to redeem coupon codes in-game for free rewards.
Here’s a list of all currently active codes.
How To Enter Codes
Tap on the “Event” icon located on the right side of the main screen.
Tap on the “New” tab.
Scroll down the list of banners until you see the “Coupon Exchange” one.
Enter your code.
The rewards will be mailed to you.