if you participated in the first Black Ops 4 beta, then access to the Blackout beta is automatically granted. It's thought to be the same client as before - it'll need an update, but having it pre-installed and ready to go could save a little time.
If you've yet to participate in the Black Ops 4 beta, to access the beta on console, you must get a Black Ops 4 beta code by pre-ordering the game. If from a store, you will receive a code from the retailer, while digital purchases will grant your account automatic access.
On PC, you download the game through
BattleNet. You don't need to pre-order to play the Black Ops 4 beta, though doing so will give you one day of early access.
Though experience won't carry over into the full game, playing the first multiplayer beta gave everyone a "unique, one-of-a-kind calling card" to carry over into the full game. It's not confirmed if this is the same, but there might be something similar.
