hello 

You have to inherit the res.partner form and add header tag if not exist.

try like below sample code.

<record id="inherit_res_partner_form" model="ir.ui.view">
    <field name="name">res.partner.form</field>
    <field name="model">res.partner</field>
    <field name="inherit_id" ref="base.view_partner_form"/>
    <field name="arch" type="xml">
        <xpath expr="//form//sheet" position="before">
            <header>
                <button name="action_name" string="Text to display" type="button_type"/> # button_type like object/action
            </header>
        </xpath>
    </field>
</record>