Here the code below can guide you;

from odoo import fields, models, ....

class Anything(models.Model):

    _name = 'model.name'
    _inherit = ['mail.thread', 'ir.needaction_mixin']

    @api.model
    def create(self, vals):
        res = super(Anything, self).create(vals)
        if vals:
            self.message_post(body=body, res)
        return res

   ####### FOR BUTTON CLICK USE

    @api.multi
    def button_send_message(self):
            for rec in self:

            body ="Message has been approved on %s", %datetime.datetime.now()
            rec.message_post(body=body)

Hi, please find below an example of posting the message as you requested:

self.message_post(cr, uid, this.id, body=("My custom message with dynamic text: %s") % (my_dynamic_text), context=context)

where 'this' is the browse object of the current record you want to post the message for.

This works if your class inherits the messaging behaviour, as you suggested in your question.
I hope this helps!