Magento 2 add sub menu into main menu

Question

How to add submenu into the main menu of the different module in admin panel

Answer 1

First of all, find out menu id of the existing module from

app/code/Vendor/ModuleOne/etc/adminhtml/menu.xml

Now use this menu id as parent to show your menu as a submenu of main module menu

<?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 action="Vendor_Module/action/index" id="Vendor_Module::my_id" module="Vendor_Module" parent="Vendor_ModuleOne::module_one_id" resource="Vendor_ModuleOne::module_one_resource" title="My SubMenu"/>
    </menu>
</config>

Answer 2

Create menu.xml file at app/code/VendorName/ModuleName/etc/adminhtml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../Magento/Backend/etc/menu.xsd">
   <menu>

        <add id="Vendor_Module::view" title="Vendor Module" module="Vendor_Module" sortOrder="11" resource="Vendor_Module::view" parent="Magento_Catalog::catalog"/>

        <add id="Vendor_Module::manage" title="Manage Module" module="Vendor_Module" sortOrder="11" parent="Vendor_Module::view" action="module/index/index" resource="Vendor_Module::manage"/>        
    </menu>
</config>

First <add> tag is about sub-menu title & second <add> tag is about sub-menu.

This menu will create under catalog menu.