You need this to get it done:
Action DetailsRequirements and how-to articles for Steam's action:
Early Access Q&A
When you indicate in Steamworks that you wish for your product to be listed in the Early Access section of Steam, you must answer the following questions:
Why Early Access?
Approximately how long will this game be in Early Access?
How is the full version planned to differ from the Early Access version?
What is the current state of the Early Access version?
Will the game be priced differently during and after Early Access?
How are you planning on involving the Community in your development process?
(Your product page and Q&A will be reviewed by Valve once they are completed.)