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 Messages"
            binding_model="library.checkout"
            res_model="library.checkout.massmessage"
            view_mode="form"
            target="new"
            />

</odoo>