Instead of a typed code, you get a “Reward Coupon” from an event leaderboard, which you exchange in the shop for items including “hot” weapons (like the Valentine or Neon series).

To find active codes, you should follow the developer's official channels where they are typically released: Official Facebook Fanpage:

No code needed for any of these — just time and consistency.

Keep an eye out for these hot deals and promotions: