Step 1: For this, you have to create a small module. In that module, you have to create di.xml on the below path where we will define the plugin.

Path : appcodeVortexCravensetcdi.xml

You have to add the below code in the 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">
	<type name="MagentoThemeBlockHtmlTopmenu">
        <plugin name="contact_us_menu" type="VortexCravensPluginTopmenu" sortOrder="1" />
    </type>
</config>

Step 2: Create a plugin on the below path and add the below-mentioned code into that file.

Path : appcodeVortexCravensPluginTopmenu.php

Code:

<?php 
namespace VortexCravensPlugin;
use MagentoFrameworkDataTreeNodeFactory;
class Topmenu
{
    protected $nodeFactory;
    protected $_storeManager;
    protected $_pageFactory;
    protected $_urlBuilder;
 
    public function __construct(
        NodeFactory $nodeFactory,
        MagentoCmsModelPageFactory $pageFactory,
        MagentoStoreModelStoreManagerInterface $storeManager,
        MagentoFrameworkUrlInterface $urlBuilder
    ) {
        $this->nodeFactory = $nodeFactory;
        $this->_pageFactory = $pageFactory;
        $this->_storeManager = $storeManager;
        $this->_urlBuilder = $urlBuilder;
    }
    public function beforeGetHtml(
        MagentoThemeBlockHtmlTopmenu $subject,
        $outermostClass = '',
        $childrenWrapClass = '',
        $limit = 0
    ) {

        $node = $this->nodeFactory->create(
            [
                'data' => [
                    'name' => __('Contact Us'),
                    'id' => 'contact-us',
                    'url' =>  $this->_urlBuilder->getUrl('contact'),
                    'has_active' => false,
                    'is_active' => false // (expression to determine if menu item is selected or not)
                ],
                'idField' => 'id',
                'tree' => $subject->getMenu()->getTree()
            ]
        );
        $subject->getMenu()->addChild($node);
    }
    
 
}

By doing these steps the contact us menu will get added in the top menu.