use WebPush\Subscription;
use WebPush\Notification;
$subscription = Subscription::createFromString('{"endpoint":"https://updates.push.services.mozilla.com/wpush/v2/AAAAAAAA[…]AAAAAAAAA","keys":{"auth":"XXXXXXXXXXXXXX","p256dh":"YYYYYYYY[…]YYYYYYYYYYYYY"}}');
$notification = Notification::create()
->withPayload('Hello world')
$statusReport = $service->send($notification, $subscription);