view.xml

<?xml version="1.0"?>
<odoo>
    <record id="view_activity_form" model="ir.ui.view">
          <field name="name">demo.activity.form</field>
          <field name="model">demo.activity</field>
          <field eval="25" name="priority"/>
          <field name="arch" type="xml">
              <form string="Demo Activity">
                <header>
                    <button name="button_activity_schedule" string="_activity schedule" type="object" class="oe_highlight"/>
                    <button name="button_activity_unlink" string="activity unlink" type="object" class="oe_highlight"/>
                    <button name="button_activity_feedback" string="activity feedback" type="object" class="oe_highlight"/>
                </header>
                <sheet>
                    <group>
                        <field name="name"/>
                        <field name="employee_id"/>
                    </group>
                </sheet>

                <div class="oe_chatter">
                    <field name="message_follower_ids" widget="mail_followers"/>
                    <field name="activity_ids" widget="mail_activity"/>
                    <field name="message_ids" widget="mail_thread"/>
                </div>

              </form>
          </field>
    </record>
</odoo>