This is my code but the Sub-Menu is created at the related main menu

 <record id="self_services_hr_main_menu" model="website.menu">
        <field name="name">HR Self Services</field>
        <field name="parent_id" ref="website.main_menu"/>
        <field name="sequence" type="int">70</field>
        <field name="website_id" ref="website.default_website"/>
</record>

<record id="self_services_hr_vacation_menu" model="website.menu">
        <field name="name">Request Vacation</field>
        <field name="parent_id" ref="custom_hr.self_services_hr_main_menu"/>
</record>