Ubisoft are accepting applications from gamers on Xbox One, PlayStation 4, and PC for the closed beta of Rainbow Six: Siege. You can register your interest here, and all names will be added to a waiting list. However, if you want to be guaranteed a place in the beta then you'll have to pre-order the game.
"Yes, you can now pre-order Tom Clancy's Rainbow Six: Siege to get access to the Closed Beta later on this year. Stay tuned for more information on the Closed Beta in the coming months," Ubisoft confirmed. You can also register for the ongoing closed alpha on PC by heading here.