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');
}