partner-contact/portal_partner_select_all/wizard/portal_wizard.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <record model="ir.ui.view" id="portal_wizard">
        <field name="model">portal.wizard</field>
        <field name="inherit_id" ref="portal.wizard_view" />
        <field name="arch" type="xml">
            <field name="user_ids" position="before">
                <group>
                    <field name="set_all_users" string="Invite all the contacts" />
                </group>
                <div
                    class="card bg-warning mb8"
                    attrs="{'invisible': [('set_all_users', '=', False)]}"
                >
                    <div class="card-body bg-warning">
                        <i
                            class="fa fa-warning"
                        /> You're about to invite all contacts listed below to the portal.
                    </div>
                </div>
            </field>
        </field>
    </record>
</odoo>