Script to request and get access token from Azure Microsoft graph API

I'm using the following script to get inactive sign in from azure:

$AppClientId="" 
$Secret = ""

$uri = "https://login.microsoft.com/a1a5384f-f0fc-4df9-9f37-caf775af43c1/oauth2/token"

$body = @{

   grant_type = "client_credentials"
   client_id = $AppClientId
   client_secret = $Secret
   resource = "https://graph.microsoft.com"

   }

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

   Write-Host  $resp.access_token




#Provide your access token. 
#$AccessToken="" 
 
#Form request headers with the acquired $AccessToken
$headers = @{'Content-Type'="application\json";'Authorization'="Bearer $AccessToken"}
 
#This request get users list with signInActivity.
$ApiUrl = "https://graph.microsoft.com/beta/users?`$select=displayName,userPrincipalName,signInActivity,userType,assignedLicenses&`$top=999"
 
$Result = @()
While ($ApiUrl -ne $Null) #Perform pagination if next page link (odata.nextlink) returned.
{
$Response =  Invoke-RestMethod -Method GET -Uri $ApiUrl -ContentType "application\json" -Headers $headers
if($Response.value)
{
$Users = $Response.value
ForEach($User in $Users)
{
 
$Result += New-Object PSObject -property $([ordered]@{ 
DisplayName = $User.displayName
UserPrincipalName = $User.userPrincipalName
LastSignInDateTime = if($User.signInActivity.lastSignInDateTime) { [DateTime]$User.signInActivity.lastSignInDateTime } Else {$null}
LastNonInteractiveSignInDateTime = if($User.signInActivity.lastNonInteractiveSignInDateTime) { [DateTime]$User.signInActivity.lastNonInteractiveSignInDateTime } Else { $null }
IsLicensed  = if ($User.assignedLicenses.Count -ne 0) { $true } else { $false }
IsGuestUser  = if ($User.userType -eq 'Guest') { $true } else { $false }
})
}
 
}
$ApiUrl=$Response.'@odata.nextlink'
}
$Result | Export-CSV "C:\LastLoginDateReport.CSV" -NoTypeInformation -Encoding UTF8

It gives the following output:

Invoke-RestMethod : The remote server returned an error: (401) Unauthorized. At D:\Work Data\Get Inactive Users from Azure AD.ps1:34 char:14

• ... Response = Invoke-RestMethod -Method GET -Uri $ApiUrl -ContentType " ...

   + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException
  

  • FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand

I tried to get the access token by Postman and import it manual to the variable and the connection is success but the problem is the token has a limited time.
So, I need to do that through the script.

Thanks