You can check and use the ASTROKINGS coupons when you go to the “Settings” menu and select “Coupon” in the game.
In the case of Apple users, the “Coupon” menu may not appear due to the App Store’s policy, so you may need to connect with an Android device or a PC emulator to use the coupon.
▶ How to use the ASTROKINGS Coupon
① Select the “Commander Info” menu at the top left of the planet view
② At the “Commander Info” menu, select “Settings” at the bottom left of the page
③ Select the “Coupon” icon on the “Settings” menu
④ Enter the coupon code at the “Coupon” menu and tap the “Redeem” button
⑤ Check the coupon rewards received through the mailbox