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

Testing your application before it is publicly released is vital to ensure quality and as few bugs as possible.

You need this to get it done:

Test Build
A build of your game for testers to play. (See "Create New Application in Steamworks" action for more details.)
"Coming Soon" Page
If you are running a closed beta, Valve strongly recommends setting up your "Coming Soon" page, as this provides a reference opportunity to test your marketing presence against the experience players are having in game. t also provides the Steam Community Hub as an easy place for your community to discuss the game and provide feedback, and makes it easy for users to Wishlist your game. (See "Build Store Page" action for more details.)
If you are running a closed beta, you will need keys for your application to give out to testers. Any customer who activates one of those keys can instantly download and play, so don’t hand them out until you’re ready. By default, users who activate a key will own the game forever, but if you want to end beta access or revoke the game from beta testers, you can revoke access using the key banning tool. See official documentation for more information on Steam keys.
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How-To Articles
A list of useful links for you:

Steamworks Documentation - Testing On Steam
Information about testing your application on Steam (enabling internal testers, setting up a closed beta, etc.)
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