Follow the instructions on that page (you don’t have to sign up to any social media stuff if you don’t want to, but they do want an email address) and it should generate a PC key for you to use on Steam. The download is around 13.5GB, and the game requires 30GB of space.
The ‘Rise Up’ beta sessions starts on 28 June (so, tomorrow) and runs until 3 July. Beta keys will not turn into the full game, unless you pre-order.
— LawBreakers (@lawbreakers) June 27, 2017