get-authtoken.ps1

$body = @{
    client_id     = $ClientId
    scope         = "https://graph.microsoft.com/.default"
    client_secret = $ClientSecret
    grant_type    = "client_credentials"
}


$uri = "https://login.microsoftonline.com/$TenantId/oauth2/v2.0/token"

$response = Invoke-RestMethod -Method post -Uri $uri -ContentType "application/x-www-form-urlencoded" -Body $body

if ($response.Access_Token) {

    Write-Output "Creating header for Authorization token"

    $script:authToken = @{
        'Content-Type'  = 'application/json'
        'Authorization' = "Bearer " + $response.Access_Token
        'ExpiresOn'     = ([timezone]::CurrentTimeZone.ToLocalTime(([datetime]'1/1/1970').AddSeconds($response.expires_on)))
        'refresh_token' = $response.refresh_token
    }
    Write-Output "Successfully authenticated!"
}
else {
    Write-Verbose $_
    Write-Error "Unable to authenticate" -ErrorAction Stop
}