https://www.pcgamer.com/fallout-76-pc-beta-date/

The Fallout 76 beta is launching on October 30, and can "only be accessed by players who pre-ordered Fallout 76 from participating retailers." Xbox players will get access a week earlier.

If you are one such player you will be able to download the 45GB beta via the Bethesda Launcher once you have entered your code into your Bethesda account. For precise instructions on how to do that, check out the Fallout 76 beta FAQ.