Odoo deals with menus by using website.menu model.

By the way, it is designed as a Nested Model.

https://en.wikipedia.org/wiki/Nested_set_model

You have to do is that just insert a new record with field parent_id here because Odoo makes up menu tree automatically.

Heres example

<?xml version="1.0" encoding="utf-8"?>
<odoo>
     <record id="submenu_item1" model="website.menu">
         <field name="name">Submenu 1</field>
         <field name="url">/page/test</field>
         <field name="parent_id" ref="website_sale.menu_shop" />
     </record>
</odoo>

See this line

<field name="parent_id" ref="website_sale.menu_shop" />

Attribute ref is an XML ID. You can search it at Settings -> Sequence & Identifiers -> External Identifiers.

Input website.menu on searching form with Model Name condition then you can see the list of menus.

Hello Silviaa,
We hope this will help you to resolve your problem.

https://apps.odoo.com/apps/modules/13.0/website_multi_menu_submenu_oin/

Thank you.
Skype: live:info_850626 OR (info@odooitnow.com)