Get access token for calling AAD secured Azure functions via PowerShell

Some time ago I posted PowerShell example which lists all Azure AD groups via Rest API: List Azure AD groups via Rest Graph API in Powershell. However if you have custom Azure functions app which is secured via AAD (see Call Azure AD secured Azure functions from C#) you may use similar approach to get access token based on client id/secret (i.e. which will use app permissions) for calling your Azure functions from PowerShell or from 3rd party clients like Postman.

In order to get access token for calling AAD secured Azure functions you may use the following PowerShell script:

param
(
	[Parameter(Mandatory=$true)]
	[string]$Tenant,
	[Parameter(Mandatory=$true)]
	[string]$ClientId,
	[Parameter(Mandatory=$true)]
	[string]$ClientSecret
)

$currentDir = $PSScriptRoot
$dllCommonDir = resolve-path($currentDir + "\Common\")

[System.Reflection.Assembly]::LoadFile([System.IO.Path]::Combine($dllCommonDir, "Microsoft.Identity.Client.dll"))

function GetAccessToken($tenant, $clientId, $ClientSecret)
{
	$appCredentials = New-Object Microsoft.Identity.Client.ClientCredential -ArgumentList $ClientSecret
	$aadLoginUri = New-Object System.Uri -ArgumentList "https://login.microsoftonline.com/"
	$authorityUri = New-Object System.Uri -ArgumentList $aadLoginUri, $Tenant
	$authority = $authorityUri.AbsoluteUri
	$redirectUri = "urn:ietf:wg:oauth:2.0:oob"
	$clientApplication = New-Object Microsoft.Identity.Client.ConfidentialClientApplication($ClientId, $authority, $redirectUri, $appCredentials, $null, $null)
	[string[]]$defaultScope = @("https://graph.microsoft.com/.default")
	$authenticationResult = $clientApplication.AcquireTokenForClientAsync($defaultScope).Result
	return $authenticationResult.AccessToken
}

Write-Host "Acquiring access token..." -foregroundcolor green
$accessToken = GetAccessToken $Tenant $ClientId $ClientSecret
if ([System.String]::IsNullOrEmpty($accessToken))
{
	Write-Host "Can't acquire access token" -foregroundcolor red
	return
}
Write-Host "Access token successfully acquired" -foregroundcolor green
$filePath = [System.IO.Path]::Combine($currentDir, [System.String]::Format("_accesstoken_{0}.txt", [System.DateTime]::Now.ToString("yyyyMMddHHmmss")))
($accessToken) | Out-File $filePath -Append

It will fetch access token and will log it to text file. For calling this script you will need Microsoft.Identity.Client.dll assembly located in Common subfolder.