public function event(string $event): self
    {
        $events = [
            'MESSAGE_RECEIVED',
            'MESSAGE_SENT',
            'MESSAGE_FAILED',
        ];
        if (!in_array($event, $events)) {
            abort(500, sprintf('Event %s not available.', $event));
        }

        $this->event = $event;

        return $this;
    }