How to add the CMS page link to the main menu in Magento 2?

Adding a CMS page link to menu in Magento 2 is possible without an extension

• Go into a category system where you can manage cms content.
• Create a cms static block and assign that static block to the category from the display setting tab
• Set Display Mode as Static block only. Do not assign any products to that category.

Or you can add a CMS page link to menu in Magento 2 by creating a custom module

Create plugin for class Magento\Theme\Block\Html\Topmenu and use before plugin on getHtml()

1. Create a module.

2. Create di.xml where you will define plugin.

<?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="Magento\Theme\Block\Html\Topmenu">
        <plugin name="add_cms_menu" type="{VendorName}{ModuleName}\Plugin\Topmenu" sortOrder="1" />
    </type>
</config> 

3. Now create the plugin class where we will add a CMS page link to the menu.

and also in this way you can add a CMS page link at navigation

<?php 
namespace {VendorName}\{ModuleName}\Plugin;
use Magento\Framework\Data\Tree\NodeFactory;
class Topmenu
{
    protected $nodeFactory;
    protected $_storeManager;
    protected $_pageFactory;
    protected $_urlBuilder;

    public function __construct(
        NodeFactory $nodeFactory,
        \Magento\Cms\Model\PageFactory $pageFactory,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Framework\UrlInterface $urlBuilder
    ) {
        $this->nodeFactory = $nodeFactory;
        $this->_pageFactory = $pageFactory;
        $this->_storeManager = $storeManager;
        $this->_urlBuilder = $urlBuilder;
    }
    public function beforeGetHtml(
        \Magento\Theme\Block\Html\Topmenu $subject,
        $outermostClass = '',
        $childrenWrapClass = '',
        $limit = 0
    ) {
        /* Showing  Cms page About us at menu */
        $page = $this->getCmspage('about-us');
        if($page == null){
            return;
        }


        $node = $this->nodeFactory->create(
            [
                'data' => [
                    'name' => $page->getTitle(),
                    'id' => $page->getIdentifier(),
                    'url' =>  $this->_urlBuilder->getUrl(null, ['_direct' => $page->getIdentifier()]),
                    '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);
    }
    protected function getCmspage($identifier){

        $page = $this->_pageFactory->create();
        $pageId = $page->checkIdentifier($identifier, $this->_storeManager->getStore()->getId());

        if (!$pageId) {
            return null;
        }
        $page->setStoreId($this->_storeManager->getStore()->getId());
        if (!$page->load($pageId)) {
            return null;
        }

        if (!$page->getId()) {
            return null;
        }

        return $page;
    }

}

Configuring, optimizing and managing a Magento 2 eCommerce store can be a daunting task. If you are having trouble creating your store in Magento 2 then you should contact Magento 2 development services. getting expert help will increase the chances of your store attracting more visitors and rankings.