oca-mrp/odoo-bringout-oca-event-event_mail/event_mail/views/event_view.xml

<?xml version="1.0" ?>
<!-- Copyright 2017 Tecnativa - Sergio Teruel <sergio.teruel@tecnativa.com>
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->
<odoo>
    <record id="view_event_form" model="ir.ui.view">
        <field name="model">event.event</field>
        <field name="inherit_id" ref="event.view_event_form" />
        <field name="arch" type="xml">
            <xpath expr="//field[@name='event_mail_ids']" position="before">
                <field
                    name="event_mail_template_id"
                    groups="base.group_user"
                    attrs="{'readonly': [('seats_expected', '>', 0)]}"
                />
            </xpath>
        </field>
    </record>
</odoo>