prepare_catalog_product_index_select

Observer skeleton

Paste into your module's etc/events.xml + create the matching Observer class. Adjust the vendor/module namespace to yours.

1. etc/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="prepare_catalog_product_index_select">
        <observer name="vendor_module_prepare_catalog_product_index_select"
                  instance="Vendor\Module\Observer\PrepareCatalogProductIndexSelect"/>
    </event>
</config>

2. Observer class

<?php
declare(strict_types=1);

namespace Vendor\Module\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;

class PrepareCatalogProductIndexSelect implements ObserverInterface
{
    public function execute(Observer $observer): void
    {
        $event = $observer->getEvent();

        // Read whatever payload this event carries. Check the
        // dispatch() call in core to see the exact keys.
        // Example: $product = $event->getProduct();

        // Your observer logic here.
    }
}

Need the full module scaffolded around this observer? Module Generator wires the events.xml, observer class, and module registration for you.