According to Pax Dei Offical Website:

For every purchase made directly via our website, we offer a bonus recipe, the Gallian Gargoyle, as a thank-you for your support.

• If you bought the game for the Pax Dei launcher, the recipe is automatically added to your account and is available for any character you create.
• If you bought a Steam key (from our website), you must link your Pax Dei account with your Steam account. Of course, it means the Pax Dei account you purchased from and the Steam account to which you added (or will add) the key. Once done, the recipe will appear alongside your other Founder’s recipes as soon as you log into the game - quit and restart the game if you linked them while in-game.

Thanks @LexaaOff for the tip!