social/mail_sendgrid/views/mail_compose_message_view.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <!-- Add sendgrid preview in compose mail wizard -->
    <record model="ir.ui.view" id="email_compose_message_sendgrid_form">
        <field name="name">mail.compose.sendgrid.form</field>
        <field name="model">mail.compose.message</field>
        <field name="inherit_id" ref="mail.email_compose_message_wizard_form"/>
        <field name="arch" type="xml">
            <field name="body" position="replace">
                <notebook>
                    <page string="Html">
                        <field name="body"/>
                    </page>
                    <page string="SendGrid Preview">
                        <field name="body_sendgrid"/>
                    </page>
                </notebook>
            </field>
        </field>
    </record>
</odoo>