Msgbox in PowerShell script run from task schedule

I have a PowerShell script that creates a schedule task to launch the script. The idea is there are some task in the script that requires reboot. At the end of the PowerShell a message box should prompt the user to let the user knows that all the tasks are completed. What am i doing wrong?

Add-Type -AssemblyName PresentationFramework

TaskName = "Run Agents Install Script"
$TaskDescription = "Run Agents Install Script at logon"
$Action = New-ScheduledTaskAction -Execute 'Powershell.exe' `
  -Argument "-executionpolicy remotesigned -File $PSScriptRoot\AgentInstall.ps1"
$Trigger =  New-ScheduledTaskTrigger -AtLogOn

Register-ScheduledTask -Action $Action -Trigger $Trigger -TaskName $TaskName -Description $TaskDescription -User "System"


$MsgBoxInput =  [System.Windows.MessageBox]::Show('Installation completed successfully.','Agent Install','OK')
Switch  ($MsgBoxInput) {
    'OK' 
   {

$MsgBoxInput =  [System.Windows.MessageBox]::Show('WARNING! Please install Imprivata agent manually if applicable.','Agent Install','OK')
   }
}

One option is to use the Terminal Services API to send a message to the console. Unfortunately, it is native API, so you need to use .NET interop to call it, but in this case it isn't too tricky:

$typeDefinition = @"
using System;
using System.Runtime.InteropServices;

public class WTSMessage {
    [DllImport("wtsapi32.dll", SetLastError = true)]
    public static extern bool WTSSendMessage(
        IntPtr hServer,
        [MarshalAs(UnmanagedType.I4)] int SessionId,
        String pTitle,
        [MarshalAs(UnmanagedType.U4)] int TitleLength,
        String pMessage,
        [MarshalAs(UnmanagedType.U4)] int MessageLength,
        [MarshalAs(UnmanagedType.U4)] int Style,
        [MarshalAs(UnmanagedType.U4)] int Timeout,
        [MarshalAs(UnmanagedType.U4)] out int pResponse,
        bool bWait
     );

     static int response = 0;

     public static int SendMessage(int SessionID, String Title, String Message, int Timeout, int MessageBoxType) {
        WTSSendMessage(IntPtr.Zero, SessionID, Title, Title.Length, Message, Message.Length, MessageBoxType, Timeout, out response, true);

        return response;
     }

}
"@

Add-Type -TypeDefinition $typeDefinition

[WTSMessage]::SendMessage(1, "Message Title", "Message body", 30, 36)

This is essentially a thin wrapper to the WTSSendMessage function.

You will need to get the SessionID via some tool like query. This script might help with that: Get-UserSession.

The TimeOut value here is 30, which means the pop-up will wait 30 seconds before returning with a value of '32000'. Set to '0' to wait forever.

The MessageBoxType is a combination of the values for uType here: MessageBox Function. So the '36' in the example is a combination of the values for 'MB_YESNO' and 'MB_ICONQUESTION', so will show a message with a question mark icon and 'yes'/'no' buttons. Note that the documentation gives the values in hexadecimal, so you'll need to convert them.

I tested this as a scheduled task running as an admin and it was able to show a message on the desktop of a different logged on user. hope it helps.