Magento 2 | Do a action before insert products by Rest API endpoint

"catalog_product_save_before" This event gets called for every product save action, including new products.

In order to use it you could do the following in your module:

**app\code\Vendor\Module\etc\webapi_rest\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="catalog_product_save_before">

            <observer name="catalog_product_save_before_check_condition" instance="Vendor\Module\Observer\Productsaveafter" />

        </event>

    </config>

Then in `Productsaveafter.php` you can implement logic to connect to your own software and perform actions like adding or updating the product. 

Example:

**app\code\Vendor\Module\Observer\Productsaveafter.php**

    <?php

      namespace Vendor\Module\Observer;

     class Productsaveafter implements ObserverInterface{    

         public function execute(\Magento\Framework\Event\Observer $observer) {

           $product = $observer->getProduct();

            // Your logic to do stuff with $product       

            }

    }

Happy Coading ... Cheers !!!!!