Cortana will soon be accessible from the Android lock screen
Once enabled in-app, Cortana will pop up as a small, round icon at the bottom of your lock screen, and, in a similar fashion to the dialer and camera shortcuts found on many devices, it can be activated by swiping up from the icon. From then on, you can get all your usual updates and information in the familiar card-like format.
Furthermore, this feature doesn't override your current lock screen, meaning that you can still use it on your device's stock lock screen or a custom one, like Microsoft's own Next Lock Screen.
If you don't feel like waiting and want to try out Cortana on your lock screen right now, you can enroll in the beta program by following the link below.