Silent push notification using davibennun/laravel-push-notification

I'm trying to send silent push notification to iOS using davibennun/laravel-push-notification. I just want to know where to add 'content-available' => 1. Here is my code:

$notification = PushNotification::Message(
            'Message.',
            array(
            'content-available' => 1,
                'custom' => array('custom data' => array(
                    'customKey'=> 'customValue',
                    'abc' => ',
            ))
        ));

via Syed Haziq Hamdani