Speaking from retail experience here, there are a few ways to get pre-orders.
Most commonly, you’ll receive the code through an e-mail that was used when purchasing (either pre-ordering online, or with a “frequent” program like GameStop’s PowerUp Rewards) that auto entitles you to the DLC content.
In the event that you don’t have an email address, you might get a code printed on your receipt. This is sometimes the distribution method for in-store purchases as well (completely side-stepping emails). Once the code is entered into an account, you are auto-entitled to the content.
Third, and most uncommon, is a printed code inside the retail box. In the event of a digital purchase, you’ll automatically get the content “bundled” with your purchase (sort of a like a digital deluxe edition).
I have no idea which of these distribution methods will be employed with Evolve, but I would assume it’s most likely the first combined with the second. But, I have no clue.