How to lock the screen in the emulator for Windows Phone 8
You can lock the screen in the emulator to test how your app responds or to test the app’s Tiles.
To lock the screen in the emulator by using the keyboard
With the emulator open, press F12 on your computer keyboard twice.
The F12 key emulates the power button on the phone. The first key press turns off the display. The second key press turns the display on again with the lock screen engaged.
You can unlock the screen by using the mouse to slide the lock screen upward.