The Call of Duty: Black Ops 4 Blackout beta will go live on PC at 10 am PT/1 pm ET on September 15, or at the same time a day earlier if you've already preordered the game. If you haven't preordered but want to get in early anyway, that can also be arranged.
Anyone who watches at least an hour of Ninja, Shroud, Summit1G, or TimTheTatman on Twitch between 10 am and 2 pm PT on September 14—during which time I assume they'll be streaming Blackout—will also be given early access to the beta. Just be sure you've linked your Battle.net account to your Twitch.tv account before you start, or else it won't count—you can do that here.
To ensure you don't waste any precious time downloading the Blackout beta when you could be playing it, you can preload it right now on Battle.net. Regardless of when you start playing, the beta will come to an end at 10 am PT/1 pm ET on September 17. Call of Duty: Black Ops 4 comes out in full on October 12.