You can find an exemple in hr_expense module.

First, create a template that you will call in your message post with source like this: 

<template id="my_new_custom_template">
    <p>The record <t t-out="name"/> has been refused</p>
    <ul>
        <li>Reason: <t t-out="reason"/></li>
    </ul>
</template>

 Then,

subtype_id = self.env['ir.model.data']._xmlid_to_res_id('mail.mt_comment')
for record in self:
   record.message_post_with_source(
       'my_module.my_new_custom_template',
       subtype_id='mail.mt_note',
       render_values={'reason': reason, 'name': record.name},
   )

I hope this helps!