All you have to do is provide your email (the one linked to your Origin account) in The Sims 4 Create A Sim Demo box on the home page. Spots in the early trial experience are limited and not everyone will be invited in. If you are selected to participate, you will receive an email invitation with full instructions at some point in the next two weeks.
Please see the FAQ for more information on The Sims 4 Create A Sim Demo.
The social simulation title will release September 2 for PC.
Source: Official Website