Hi,
This is an example for adding a search panel to the sale module tree view in Odoo15.

<odoo>

    <record id="sale_order_search_panel_sale_order_search" model="ir.ui.view">

        <field name="name">sale.order.search (module_name)</field>

        <field name="model">sale.order</field>

        <field name="inherit_id" ref="sale.view_sales_order_filter"/>

        <field name="arch" type="xml">

            <search position="inside">

                <searchpanel>

                    <field name="invoice_status" enable_counters="1"/>

                    <field name="state" enable_counters="1"/>

                    <field name="partner_id" enable_counters="1"/>

                </searchpanel>

            </search>

        </field>

    </record>

</odoo>

Hope it helps