Method for activating the game isn't obvious.
1. Sign Up at Bohemia's website(link in original post)
2. Validate the email they send you and you'll be taken to your account's page.
Scroll to the bottom and click 'Connect' to your Steam account
3. It'll pop up a Steam confirmation window. Sign In
4. Go back to the original link in original posthttps://store.bistudio.com/products/project-argo
5. Click on the green Steam button to own the game
6. Click on blue button where your name is (see attached picture)
7. Click on 'Order Details' button on Project Argo (see attached picture)
Select 'Redeem on Steam' button
8. Project Argo (Prototype) will appear in your Steam Library now