Action Details
Requirements and how-to articles for Steam's action:

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Keys are obtained by requesting them in the Steamworks portal. To request keys, click the "Request Steam Product Keys" button on your application's landing page in Steamworks. If you don't see that button, your account doesn’t have “Generate Steam Keys” permission. (Note: Requests for large amounts of keys will be manually reviewed by Valve before processing.)
Look for reviewers/influencers with large followings who post online about games which they play (e.g. writing reviews/articles, recording videos, live streaming.) Send them a message with a brief description of your game and the key for them to activate on their Steam account.
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How-To Articles
A list of useful links for you:

Steamworks Documentation - Steam Keys
Information about keys, including rules and guidelines for distributing them.
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