This key is redeemed and then you're registered to take part in the beta once it begins, which won't be "until a later, yet-to-be announced date." A new Wolfenstein trailer is out.
The inclusion of a beta for DOOM suggests they'll be implementing a multiplayer component, as its unlikely they'd want external testers for the single player portion.
"An insert will be included inside your boxed copy of Wolfenstein: The New Order," reads the . "This insert will include a key code along with code redemption instructions. Once the key code is redeemed, you'll be registered to participate in the DOOM beta once it begins. Check back at wolfenstein.com/doom for updated information."
"Although you'll receive the DOOM beta key code with your copy of Wolfenstein: The New Order, the DOOM beta won't begin until a later, yet-to-be announced date. We will contact you with news and updates surrounding the start of the DOOM beta via the email address provided during the key code redemption process."
Wolfenstein: The New Order releases on PC, Xbox 360, PS3, Xbox One and PS4 May 20th in the US, 23rd in EU.