https://www.gamespot.com/articles/fallout-76-beta-will-be-preceded-by-a-stress-test-/1100-6462426/

Ahead of the full game's release date in November, Bethesda will host a Fallout 76 beta later this month. The first leg of this will begin on October 23 for Xbox One, but you may be able to put the game through its paces even sooner than that. Bethesda has announced that before the beta launches, some Xbox Insiders will be able to take part in a limited stress test to help the studio prepare for the beta launch.

Participants for the stress test will be chosen from a pool of players who have both pre-ordered Fallout 76 for Xbox One, and who are also members of the Xbox Insider program. Fulfilling those two criteria doesn't guarantee you a spot, but it does put you in the running. Starting on October 13, you can check if you've been given access to the stress test by looking at the Insider Content tab of the Xbox Insider program.