Fallout 76, both the full retail release and the upcoming B.E.T.A., will only be available via Bethesda.net, meaning it will not be coming to Steam at launch.
In the Fallout 76 B.E.T.A. FAQ that announced that the Break-It Early Test Application will most likely be the full game and your progress will carry over, Bethesda also mentions that the B.E.T.A. will be available on “Xbox One, PlayStation 4, and on PC (via Bethesda.net).”
Source: IGN Video Games All