Your di.xml code will be look like:
<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<preference for="Magento\Sitemap\Model\ResourceModel\Catalog\Product" type="{Vendorname}\{Modulename}\Rewrite\Magento\Sitemap\Model\ResourceModel\Catalog\Product"/>
</config>
And Rewrite class code:
<?php
namespace {Vendorname}\{Modulename}\Rewrite\Magento\Sitemap\Model\ResourceModel\Catalog;
class Product extends \Magento\Sitemap\Model\ResourceModel\Catalog\Product
{
protected function _prepareProduct(array $productRow, $storeId)
{
// Do changes at here
$product = new \Magento\Framework\DataObject();
$product['id'] = $productRow[$this->getIdFieldName()];
if (empty($productRow['url'])) {
$productRow['url'] = 'catalog/product/view/id/' . $product->getId();
}
$product->addData($productRow);
$this->_loadProductImages($product, $storeId);
return $product;
}
}