Hi,

In the model you can inherit this,

    _inherit = ['mail.thread', 'mail.activity.mixin']

Then in the XML,

<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" options="{'post_refresh': 'recipients'}"/>
</div>

You can have a look at this video: How to Add Chatter to Form View in Odoo

Thanks