GameStop will have a "limited" supply of codes to hand out, starting April 1.
Looking to check out the upcoming Doom beta but don't have a code? Retailer GameStop has a solution. The company has announced that, starting April 1 at 6 PM EST, all you have to do is go to any GameStop store and ask for a code and you'll get one.
A GameStop representative tells GameSpot the codes will be "limited in supply" and will be handed out on a first-come, first-served basis. You can find a GameStop store near you here.

We also confirmed with GameStop that codes will be available on all Doom platforms: PlayStation 4, Xbox One, and PC.

As announced earlier this week, the Doom closed multiplayer beta runs March 31 through April 3. Players who preordered Wolfenstein: The New Order and redeemed their beta code will have access to the closed beta period, though it's not immediately clear if there are also other ways (outside of this and the GameStop promotion) to get in. More information on can be found on the official website.