web/web_advanced_search/static/src/xml/web_advanced_search.xml

<?xml version="1.0" encoding="UTF-8"?>
<!-- Copyright 2017-2018 Jairo Llopis <jairo.llopis@tecnativa.com>
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->
<templates>
    <t t-extend="SearchView.FilterMenu">
        <t t-jquery=".o_filters_menu" t-operation="append">
            <li class="divider"/>
            <li>
                <a class="o_add_advanced_search">Add Advanced Filter</a>
            </li>
        </t>
    </t>
</templates>