Redeem Steam key to account with Steam OpenID like Humble Bundle

Humble Bundle has started requiring the user to sign in through Steam's OpenID authentication where it then redeems the Steam key on the user's account. I was unable to find any documentation on this

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How to Buy a Humble Bundle and Redeem It on Steam

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How to Buy a Humble Bundle and Redeem It on Steam

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How to Buy a Humble Bundle and Redeem It on Steam

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