Register Windows Startup/Shutdown script

Register.ps1

function Register-EventScript {
    param (
        [string] $eventToRegister, # Either Startup or Shutdown
        [string] $pathToScript,
        [string] $scriptParameters
    )
    
    $path = "$ENV:systemRoot\System32\GroupPolicy\Machine\Scripts\$eventToRegister"
    if (-not (Test-Path $path)) {
        # path HAS to be available for this to work
        New-Item -path $path -itemType Directory
    }

    # Add script to Group Policy through the Registry
    "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Group Policy\Scripts\$eventToRegister\0\0",
    "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Group Policy\State\Machine\Scripts\$eventToRegister\0\0" |
    ForEach-Object { 
        if (-not (Test-Path $_)) {
            New-Item -path $_ -force
        }
    }

    "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Group Policy\Scripts\$eventToRegister\0",
    "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Group Policy\State\Machine\Scripts\$eventToRegister\0" |
    ForEach-Object {
        New-ItemProperty -path "$_" -name DisplayName -propertyType String -value "Local Group Policy" -force
        New-ItemProperty -path "$_" -name FileSysPath -propertyType String -value "$ENV:systemRoot\System32\GroupPolicy\Machine"  -force
        New-ItemProperty -path "$_" -name GPO-ID -propertyType String -value "LocalGPO" -force
        New-ItemProperty -path "$_" -name GPOName -propertyType String -value "Local Group Policy" -force
        New-ItemProperty -path "$_" -name PSScriptOrder -propertyType DWord -value 2  -force
        New-ItemProperty -path "$_" -name SOM-ID -propertyType String -value "Local" -force
    }
    "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Group Policy\Scripts\$eventToRegister\0\0",
    "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Group Policy\State\Machine\Scripts\$eventToRegister\0\0" |
    ForEach-Object {
        New-ItemProperty -path "$_" -name Script -propertyType String -value $pathToScript -force 
        New-ItemProperty -path "$_" -name Parameters -propertyType String -value $scriptParameters -force
        New-ItemProperty -path "$_" -name IsPowershell -propertyType DWord -value 1 -force
        New-ItemProperty -path "$_" -name ExecTime -propertyType QWord -value 0 -force
    }
}

# get crrentlocation so we can form the full path to the script to register
$currentLocation = $PSScriptRoot

# register the script twice
Register-EventScript -eventToRegister "Startup" -pathToScript "$currentLocation\ScriptToRun.ps1" -scriptParameters "OnStartup"
Register-EventScript -eventToRegister "Shutdown" -pathToScript "$currentLocation\ScriptToRun.ps1" -scriptParameters "OnShutdown"

ScriptToRun.ps1

param (
        [string] $message
)

function Run-Script {
    param (
        [string] $message
    )
    # get the date in our format
    $date = Get-Date
    Get-date $date -f "yyyyMMdd HH:mm:ss"
    # log the date/time and the message
    "$($date.ToUniversalTime().ToString("yyyyMMdd HH:mm:ss")) - $message" >> "C:\Temp\Log.txt"
}

# call the script with the incoming parameter
Run-Script -message $message

Working

The Register script will register the ScriptToRun.ps1 for both Machine Startup and Shutdown.
The ScriptToRun will then log those events to a text file for easy testing.

Tested

Tested on a Windows 10 VM (1909)

Caveat:

You need to run the register script with an elevated session