woocommerce_custom_order.md

$order = wc_create_order();
$order_id = trim(str_replace('#', '', $order->get_order_number()));
update_post_meta($order_id, '_customer_user', $user_id);

$linked_product = (int)get_post_meta( $course_id, 'linked_product', true );

$order->add_product( get_product($linked_product), 1); // This is an existing SIMPLE product

$coupon_code = $rec_coupon;
$coupon = new WC_Coupon($coupon_code);
$coupon_post = get_post($coupon->id);
if($coupon_post) {
    $discount_total = $coupon->get_amount();
    $item = new WC_Order_Item_Coupon();
    $item->set_props(array('code' => $coupon_code, 'discount' => $discount_total, 'discount_tax' => 0));
    $order->add_item($item);
    $order->save();

    foreach ($order->get_items() as $order_item) {
        $total = $order_item->get_total();
        $order_item->set_subtotal($total);
        $calc_total = $total - $discount_total;
        $calc_total = ($calc_total > 0) ? $calc_total : 0;
        $order_item->set_total($calc_total);
        $order_item->save();
    }
    $order->save();
}
$order->calculate_totals();

$order->update_status("completed", 'order_note', true);