social/mail_partner_opt_out/views/res_partner_views.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <record id="res_partner_view_form_inherit_mail" model="ir.ui.view">
        <field name="name">res.partner.view.form.inherit.mail</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form" />
        <field name="arch" type="xml">
            <field name="email" position="before">
                <button
                    name="mail_blacklist_add"
                    class="fa fa-plus text-success"
                    title="Click to add this email to the blacklist."
                    type="object"
                    groups="base.group_user"
                    attrs="{'invisible': [('is_blacklisted', '=', True)]}"
                />
            </field>
        </field>
    </record>
</odoo>