invoke-webrequest.ps1

$sourcepath      = 'https://installer.msi'
$destinationpath = "C:temppath"
$filename        = "installer.msi"
$registry        = "hklm:softwareapplicationname"
$logname         = "eventlogname"
$logsource       = "eventlogsource"

if (-not (Get-EventLog -list | Where-Object {$_.logdisplayname -eq $logname})) {
	New-EventLog -LogName $logname -Source $logsource
}

$wp = New-Object Security.Principal.WindowsPrincipal([System.Security.Principal.WindowsIdentity]::GetCurrent())
if (-not $wp.IsInRole([Security.Principal.WindowsBuiltinRole]::Administrator)) {
	Write-Eventlog  -Logname $logname -Source $logsource -EventID 1000 -EntryType Error -Message "File downloader-script requires administrative privileges, please re-launch with elevated credentials."
	#exit
}

if (-not (test-path($destinationpath))){
	New-Item -ItemType Directory -Force -Path ($destinationpath)
}
if (-not (test-path($registry))){
	New-Item -Path hklm:software -Name applicationname
}

if((Get-ItemProperty $registry -Name "etag" -ErrorAction 0).etag) {
	$etag = (Get-Itemproperty $registry -Name "etag").etag
}else{ 
	$etag = "0x0x0"
	Set-ItemProperty -Path $registry  -Name "etag" -Value $etag
}

try {
	$abb = Invoke-WebRequest -Uri $sourcepath -Headers @{"If-None-Match"=$etag} 

	[io.file]::WriteAllBytes(($destinationpath+$filename), $abb.Content)
	$etag = $abb.headers.etag
	Set-ItemProperty -Path $registry -Name etag -Value $etag
}
catch {
}