odoo16/addons/mass_mailing/wizard/mail_compose_message_views.xml

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

        <!-- Add mass mail campaign to the mail.compose.message form view -->
        <record model="ir.ui.view" id="email_compose_form_mass_mailing">
            <field name="name">mail.compose.message.form.mass_mailing</field>
            <field name="model">mail.compose.message</field>
            <field name="inherit_id" ref="mail.email_compose_message_wizard_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='notify']" position="after">
                    <field name="campaign_id" groups="mass_mailing.group_mass_mailing_campaign"
                        attrs="{'invisible': [('composition_mode', '!=', 'mass_mail')]}"/>
                    <field name="mass_mailing_name"
                        attrs="{'invisible': [('composition_mode', '!=', 'mass_mail')]}"/>
                </xpath>
                <xpath expr="//button[@name='action_send_mail'][not(hasclass('o_mail_send'))]" position="attributes">
                    <!-- 'Log' button -->
                    <attribute name="attrs">
                        {'invisible': [
                            '|',
                                ('is_log', '=', False),
                                '&amp;',
                                    ('mass_mailing_name', '!=', ''),
                                    ('mass_mailing_name', '!=', False)
                        ]}
                    </attribute>
                </xpath>
                <xpath expr="//button[hasclass('o_mail_send')]" position="attributes">
                    <!-- 'Send' button -->
                    <attribute name="attrs">
                        {'invisible': [
                            '|',
                                ('is_log', '=', True),
                                '&amp;',
                                    ('mass_mailing_name', '!=', ''),
                                    ('mass_mailing_name', '!=', False)
                        ]}
                    </attribute>
                </xpath>
                <xpath expr="//button[@name='action_send_mail']" position="after">
                    <button string="Send Mass Mailing" name="action_send_mail" type="object" class="btn-primary o_mail_send"
                        attrs="{'invisible': ['|', ('mass_mailing_name', '==', ''), ('mass_mailing_name', '==', False)]}" data-hotkey="q"/>
                </xpath>
            </field>
        </record>

</odoo>