private static function makeRequest($key, $secret, $account)
    {
        $credentials = base64_encode($key . ':' . $secret);
        $endpoint = Core::instance()->getEndpoint(MPESA_AUTH, $account);

        return Core::instance()->client()->request('GET', $endpoint, [
            'headers' => [
                'Authorization' => 'Basic ' . $credentials,
                'Content-Type'  => 'application/json',
            ],
        ]);
    }