social/mail_tracking_mailgun/views/mail_tracking_email.xml

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

    <record id="mailgun_manual_check" model="ir.ui.view">
        <field name="name">Manual Mailgun check</field>
        <field name="model">mail.tracking.email</field>
        <field name="inherit_id" ref="mail_tracking.view_mail_tracking_email_form" />
        <field name="arch" type="xml">
            <field name="state" position="before">
                    <button
                    name="action_manual_check_mailgun"
                    type="object"
                    string="Re-sync Mailgun"
                />
            </field>
        </field>
    </record>

</odoo>