Magento 2 : How to add custom Menu with sub-menu in admin?

Create Below file in your module
app/code/Vendor/Module/etc/adminhtml/menu.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../Backend/etc/menu.xsd">
    <menu>
        <add id="Vendor_module::Vendor_module" title="Menu Title" module="Vendor_module"  sortOrder="20" dependsOnModule="Vendor_module" resource="Vendor_module::module"/>
        <add id="Vendor_module::Vendor_module_index" title="Sub Menu title" module="Vendor_module"  sortOrder="20" dependsOnModule="Vendor_module" action="Admin_Controller_path" resource="Vendor_module::Vendor_module_index" parent="Vendor_module::Vendor_module"/>
    </menu>
</config>