pos/pos_invoice_send_mail/views/pos_config.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <record id="pos_config_view_form" model="ir.ui.view">
        <field name="model">pos.config</field>
        <field name="inherit_id" ref="point_of_sale.pos_config_view_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='invoice_journal_id']/parent::div" position="after">
                <div class="row">
                    <label for="iface_invoice_mail" class="col-lg-3 o_light_label"/>
                    <field name="iface_invoice_mail"/>
                </div>
                <div class="row" attrs="{'invisible': [('iface_invoice_mail', '=', False)]}">
                    <label string="Email Template" for="invoice_mail_template_id" class="col-lg-3 o_light_label"/>
                    <field name="invoice_mail_template_id" attrs="{'required': [('iface_invoice_mail', '=', True)]}"/>
                </div>
            </xpath>
        </field>
    </record>

</odoo>