If you have updated your touch/slate device to Windows 8.1 Update you will have noticed that the Search button has appeared on your start menu but not the power button. If you were wondering how to enable this button then unfortunately there is no easy way to do this as it is dictated by the type of hardware that you are running the device on (see http://blogs.technet.com/b/askds/archive/2014/04/07/options-for-managing-go-to-desktop-or-start-after-sign-in-in-windows-8-1.aspx ). Meaning if your device is a classified as a touch "slate" device then the Power Button will not be visible. But as discovered by @teroalhonen there is a registry key that can be configured to override this setting:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell\Launcher\ DWORD Launcher_ShowPowerButtonOnStartScreen Value 1 = Button Enabled Value 0 = Button Disabled

As this is as simple as a registry key then you can easily implemented it as a Group Policy Preferences Registry Extension setting. To do this open a Group Policy Object that targets the users that you want to enable/disable the power button for and then navigate to User Configuration > Preferences > Windows Settings then right click Registry and go to New > Registry Item (see below) Then type in "Software\Microsoft\Windows\CurrentVersion\ImmersiveShell\Launcher\" into the Key Path field and "Launcher_ShowPowerButtonOnStartScreen" into the Value Name box. After that Select REG_DWORD and value "1" (enable) or "0" (disable) to configured the power button. Once done you need to wait for a Group Policy Update and then the user to log off and back on for the power button to appear or disappear. Source @teroalhonen (Via http://winsupersite.com/windows-8/windows-81-update-1-tip-toggle-power-button )