Messagebox and action if not pressed

Schnittlauch 

In the case of a GPO I think the drawback is that will shutdown the computers even if they are at home, and the goal is just to turn off the computers which are in the office.

Meanwhile I got some code to test, to check if it works for the goal. The code is this one:

# How long to display the dialog before it closes automatically.
$timeoutSecs = 3 * 60 # 30 minutes.
# Translate this into the time of day, to show to the user.
$timeoutTimeOfDay = (Get-Date).AddSeconds($timeoutSecs).ToString('T')

# Put up a message box with a timeout and OK / Cancel buttons.
$response = 
  (New-Object -ComObject WScript.Shell).Popup(
    @"
Your computer will shut down at $timeoutTimeOfDay.

Press OK to shut down now, or Cancel to cancel the shutdown.
"@, 
    $timeoutSecs, 
    'Pending shutdown', 
    49  # 48 + 1 == exclamation-point icon + OK / Cancel buttons
  )

if ($response -eq 2) { # Cancel pressed.
  Write-Warning 'Shutdown aborted by user request.'
  exit 2
}

# OK pressed ($response -eq 1) or timed out ($response -eq -1), 
# proceed with shutdown.
'Shutting down...' 
# Stop-Computer -Force