Hi IlyaMuromets! Unfortunately, you can only add 1 Steam key on an itch product, so the only Steam key we were able to associate with a game purchase was the game itself. The extras will have to be downloaded from itch and not Steam.
Viewing post in The Symbiant - Gay Yaoi (18+) comments
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If you would rather get the games on Steam, I suggest you grab them there.
https://store.steampowered.com/app/1883090/The_Symbiant/
https://store.steampowered.com/app/2355140/The_Symbiant_ReUnion/