Get-CorpMicrosoftGraphAccessToken.ps1

# Connects to Azure Active Directory to authenticate and retrieve a valid access token using a Client ID and secret
function Get-CorpMicrosoftGraphAccessToken
{
   param(
	[string] $TenantId,
	[string] $ClientId,
	[string] $ClientSecret
   )
	
    $AccessToken = ""
    $Method = "POST"
    $ContentType = "application/x-www-form-urlencoded"
    $Uri = "https://login.microsoftonline.com/$($TenantId)/oauth2/v2.0/token"
    $Body = "client_id=$($ClientId)&scope=https%3A%2F%2Fgraph.microsoft.com%2F.default&client_secret=$($ClientSecret)&grant_type=client_credentials"
    
    try
    {
        $Response = Invoke-WebRequest -Uri $Uri -Method $Method -ContentType $ContentType -Body $Body -UseBasicParsing
        $Content = ConvertFrom-Json $Response.Content
        $AccessToken = $Content.access_token

        Write-Host "Access token aquired from Microsoft Graph"
    }
    catch
    {
        Write-Error "Unable to aquire access token from Microsoft Graph"
    }

    return $AccessToken
}