How to include API Subscription key in my Laravel End Point

In my Laravel-5.8, I am trying to consume API end point GET Request with Guzzle:

  use App\Employee;
  use App\Department;

  public function index() 
  {  
      $client = new GuzzleHttp\Client();
       $res = $client->request('GET','https://api.employees.net/allemployees')->getBody();

        $clientdatas = json_decode($res, true);

     ...
  }

The API has subscription key.

In POSTMAN, I did it this way and it works:

GET        https://api.employees.net/allemployees?key=fgfdhsddddDDDDD

How do I add the subscription key to my Laravel api

  public function index() 
  {  
      $client = new GuzzleHttp\Client();
       $res = $client->request('GET','https://api.employees.net/allemployees')->getBody();

        $clientdatas = json_decode($res, true);

     ...
  }

Thank you.