whatsapp_web_groups/change_vat_in_partner/views/res_partner_view.xml

<?xml version='1.0' encoding='utf-8'?>
<odoo>
    <record model="ir.ui.view" id="res_partner_view_change_vat">
        <field name="name">res.partner.view.change.vat</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='vat']" position="replace">
                <field name="vat" placeholder="e.g. BE0477472701"/>
            </xpath>
        </field>
    </record>
</odoo>