library/library_checkout/wizard/checkout_mass_message_wizard.xml

<?xml version="1.0" ?>
<odoo>
    <record id="view_form_checkout_message" model="ir.ui.view">
        <field name="name">Library Checkout Mass Message Wizard</field>
        <field name="model">library.checkout.massmessage</field>
        <field name="arch" type="xml">
            <form>
                <group>
                    <field name="message_subject"/>
                    <field name="message_body"/>
                    <field name="checkout_ids"/>
                </group>
                <footer>
                    <button type="object"
                            name="button_send"
                            string="Send Message"
                            />
                    <button special="cancel"
                            string="Cancel"
                            class="btn-secondary"
                            />
                </footer>
            </form>
        </field>
    </record>
    <act_window id="action_checkout_message"
                name="Send Message"
                binding_model="library.checkout"
                res_model="library.checkout.massmessage"
                view_mode="form"
                target="new"
    />
</odoo>