Windows 8 power button on start screen

To enable the power button on the start screen in Windows 8.1 (disabled by default on screens smaller than 8.5″), add either of the following registry keys:

For current user:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell

For all users on the device:

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell

Value to add:

Launcher_ShowPowerButtonOnStartScreen,  DWORD = 1

Note: if setting this value via Group Policy Editor, the HKCU setting will only work if applying to user objects and the HKLM will only apply to computer objects – so be sure to apply the correct setting to the correct OU.

Ref: https://blogs.technet.microsoft.com/askpfeplat/2014/04/06/exploring-windows-8-1-update-start-screen-desktop-and-other-enhancements/