Unova RPG appears to be a fan-made Pokémon MMORPG (browser-based or client-based). Since it's not an official Nintendo/Game Freak product, there is no universal coupon system—it depends entirely on the specific server/website you're playing on.
If you tell me the exact website/server name (e.g., "PokeOne," "Pokemon Revolution Online," "UnovaRPG.com"), I can check if they have a known code system.