https://www.gamespot.com/articles/fallout-76-beta-launches-in-october-will-start-out/1100-6460660/

Fallout 76 is going to offer a beta exclusively to those who pre-order the game. But even if you're among those who are already changing into your jumpsuits and calibrating your Pip-Boys, you won't be venturing into the wasteland of West Virginia for a few more months.

A Bethesda forum post shed more light on the timeline. It says that it will begin selecting participants for the beta from the pool of pre-orders starting in October. The plan is to start small and expand over time, so you may not make it in right away. If you ordered on Xbox One, you have a slight leg up on the other platforms. You can check out the FAQ for more details on how to redeem your code.