Create file: `etc/events.xml`

    <event name="sales_order_place_after">
        <observer name="sales_order_place_after_event" instance="Vendor\Module\Observer\SalesOrderPlaceAfter" />
    </event>

> Create `Vendor/Module/Observer/SalesOrderPlaceAfter.php`

    public function execute(Observer $observer)
    {
        /** @var Order $order */
        $order = $observer->getData('order');
        $payment = $order->getPayment();
        $order->setState('new');
        $order->setStatus('pending');
    }