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

Steam keys are free and can be activated by customers on Steam to grant a license to a product. Giving out keys for your game to reviewers and influencers means they are more likely to play your game and help spread the word to potential customers.

You need this to get it done:

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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