create di.xml file in your custom module

    /magento2/app/code/Contact/Custom/etc/di.xml 

add below code in di.xml file.

    <?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\Contact\Controller\Index\Post" type="Contact\Custom\Controller\Index\ExtendIndex" />
    </config>

create ExtendIndex.php file `/magento2/app/code/Contact/Controller/Controller/Index/ExtendIndex.php`

    <?php
         namespace Contact\Custom\Controller\Index;
    class ExtendIndex extends \Magento\Contact\Controller\Index\Post
    {
        /**
         * Index action
         *
         * @return $this
         */
        public function execute($coreRoute = null)
        {
             
            
            $this->messageManager->addSuccess('Message from new controller.');
            return parent::execute($coreRoute);
        }
    }