https://www.polygon.com/2018/8/7/17660444/fallout-76-beta-steam-release-windows-pc

As the Fallout 76 beta nears its October launch, Bethesda has provided additional details about the invite-only trial. That includes where Windows PC players will be able to access the beta: only on Bethesda’s exclusive launcher, and not on the popular Steam platform.

“The B.E.T.A. will be available on Xbox One, PlayStation 4 and on PC (via Bethesda.net),” reads the test build’s FAQ.