Bethesda has still said nothing about when the Fallout 76 beta will begin (aside from some time before the full game's November 10 release date) or how long it will last. But now we know that, whenever it does begin, players on Xbox One will be the first ones to exit Vault 76 and start exploring the mostly-non-radioactive hills and valleys of West Virginia. Um, not that that will present a huge advantage or anything - chances are very slim that progress will carry over from the beta into the full game. But there's still something fun about being first.
If you were hoping to use the Fallout 76 beta as a chance to test out the game for free, it sounds like you may already be out of luck. According to the FAQ, pre-ordering Fallout 76 is "the only way to get access to the beta" (emphasis added). Bethesda will probably do some code giveaways via contests and stuff like that, but you can't sign up ahead of time for a shot at a beta slot. If you want guaranteed access, you'll need to put some money down ahead of time.