[Update: I had missed this earlier, but according to the official FAQ, all progress made in the Fallout 76 beta will carry over to the full game. According to Bethesda, the beta actually is the code for the final release, so you’ll be able to continue where you left off once the game launches.]
Fallout 76’s beta will begin in late October. True to its word, Bethesda will launch the beta on Xbox One, first, on October 23. PC and PS4 users will be able to jump in on October 30. It seems the beta will run until launch, though an end date wasn’t given. Bethesda is saying to keep an eye out on the official Fallout Twitter page for updates regarding beta server status and when it will be ready for play.
Along with that info, Bethesda has uploaded the intro cinematic for Fallout 76. If you want to keep it a surprise for launch, I’d recommend not clicking the video below. It doesn’t show much that wasn’t already seen during E3, though.