pos/pos_backend_partner/views/backend_partner.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <record id="view_partner_tree" model="ir.ui.view">
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_tree"/>
        <field name="arch" type="xml">
            <field name="display_name" position="before">
                    <button string="Choose"
                        type="object" name="select_in_pos_current_order"
                        help="Select as customer in point of sale"/>
                </field>
        </field>
    </record>

    <record id="action_select_partner_pos" model="ir.actions.act_window">
        <field name="name">Client</field>
        <field name="type">ir.actions.act_window</field>
        <field name="res_model">res.partner</field>
        <field name="view_mode">tree,form</field>
        <field name="domain">[('customer', '=', True)]</field>
        <field name="context">{}</field>
    </record>

</odoo>