Get-MSGraphOAuthToken.ps1

Function Get-MSGraphOAuthToken {
     <#

    .SYNOPSIS
    Acquire authentication token for MS Graph API

    .DESCRIPTION
    If you have a registered app in Azure AD, this function can help you get the authentication token
    from the MS Graph API endpoint. Each token is valid for 60 minutes.

    .PARAMETER ClientID
    This is the registered Application ID in AzureAD

    .PARAMETER ClientSecret
    This is the secret key of the registered app in AzureAD

    .PARAMETER TenantID
    This is your Office 365 Tenant ID/Domain

    .EXAMPLE
    $oauth = Get-MSGraphOAuthToken -ClientID $ClientID -ClientSecret $ClientSecret -TenantID $TenantID

    The above example gets a new token using the ClientID, ClientSecret and TenantID combination

    .NOTES
    The access token is stored in the Header property of the returned object
    #>

    param (
        [Parameter(Mandatory, Position = 0)]
        [ValidateNotNullOrEmpty()]
        [string]
        $ClientID,

        [Parameter(Mandatory, Position = 1)]
        [ValidateNotNullOrEmpty()]
        [string]
        $ClientSecret,

        [Parameter(Mandatory, Position = 2)]
        [ValidateNotNullOrEmpty()]
        [string]
        $TenantID
    )

    # Build the POST Request Parameters
    $request = @{
        Method = 'POST'
        URI    = "https://login.microsoftonline.com/$TenantID/oauth2/v2.0/token"
        body   = @{
            grant_type    = "client_credentials"
            scope         = "https://graph.microsoft.com/.default"
            client_id     = $ClientID
            client_secret = $ClientSecret
        }
    }

    try {
        # Request for OAuth
        $oauth = Invoke-RestMethod @request -ErrorAction STOP

        # Add the Header property to OAuth
        $oauth | Add-Member -MemberType NoteProperty -Name Header -Value @{'Authorization' = "$($oauth.token_type) $($oauth.access_token)" }

        # Add the OAuthStart property to OAuth - Token valid start time
        $oauth | Add-Member -MemberType NoteProperty -Name Auth_Start -Value (Get-Date)

        # Add the OAuthEnd property to OAuth - Token expiration time
        $oauth | Add-Member -MemberType NoteProperty -Name Auth_End -Value (Get-Date).AddSeconds($oauth.expires_in)

        # Return the $oauth object
        return $oauth
    }
    catch {
        Write-Output $_.Exception.Message
        Break
    }
}

<# EXAMPLE
$ClientID = 'client-id'
$ClientSecret = 'client-secret'
$TenantID = 'tenant-id'

$oauth = Get-MSGraphOAuthToken $ClientID $ClientSecret $TenantID
$oauth.Header
#>