In this guide, we are going to show you how to create a custom admin menu in Magento 2.

The first step is to create a module.

Let’s give the module a name. Call it: Magemonkey_Newadminmenu

Then, create registration.php file

<?php
MagentoFrameworkComponentComponentRegistrar::register(
    MagentoFrameworkComponentComponentRegistrar::MODULE,
    'Magemonkey_Newadminmenu',
    __DIR__
);

create module.xml file in etc/

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
            <module name="Magemonkey_Newadminmenu" setup_version="1.0.0"></module>
</config>

create menu.xml file in etc/adminhtml/

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Backend:etc/menu.xsd">

    <menu>
        <add id="Magemonkey_Newadminmenu::first_menu"
             title="My Main Menu"
             module="Magemonkey_Newadminmenu"
             sortOrder="20"
             resource="Magento_Backend::content" />

        <add id="Magemonkey_Newadminmenu::second_menu"
             title="My Sub Menu"
             module="Magemonkey_Newadminmenu"
             sortOrder="1"
             action="newadminmenu/index/index"
             parent="Magemonkey_Newadminmenu::first_menu"
             resource="Magento_Backend::content" />

    </menu>
</config>

Know the description for element variables in code:

id: The unique identifier for the custom admin menu
title: The title will be shown as a menu name
Module: Magemonkey_Newadminmenu (name of the current module)
sortOrder: Set custom admin menu order
resource: The rule for admin users can access the custom admin menu
Action: Link for admin controller URL
parent: Name of the parent menu which custom menu depends on sub menu

After running this command for install that module

php bin/magento setup:upgrade
php bin/magento cache:clean

Now It will be shown in admin menu as seen in the below image.