flectra/addons/mass_mailing_event/views/event_views.xml

<?xml version="1.0"?>
<flectra>
    <record id="event_event_view_form_inherit_mass_mailing" model="ir.ui.view">
        <field name="name">event.event.view.form.inherit.mass.mailing</field>
        <field name="model">event.event</field>
        <field name="inherit_id" ref="event.view_event_form"/>
        <field name="arch" type="xml">
            <button name="%(event.act_event_registration_from_event)d" position="after">
                <button class="oe_stat_button" type="object"
                        name="action_mass_mailing_attendees"
                        icon="fa-envelope-o"
                        attrs="{'invisible': [('seats_expected', '=', 0)]}">
                    Mail Attendees
                </button>
            </button>
        </field>
    </record>
</flectra>