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

Enhanced_Steam_Firefox/changelog.txt at master · jshackles

13 simple ways to redeem Steam Games automatically (I made a

Does Humble Bundle give Steam keys & how to redeem them

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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Account got Hijacked - Please DON'T make my mistake!

How to Buy a Humble Bundle and Redeem It on Steam