Hi,

In Odoo 15, you can create a task when a message is created by using the mail thread feature and the mail.message model. Here is an example of a new model that creates a task when a message is created:

class MyModel(models.Model):
    _inherit = ['mail.thread', 'mail.message']

    @api.model
    def create(self, vals):
 
        task = self.env['project.task'].create({
            'name': vals.get('subject', ''),
            'user_id': vals.get('author_id', False),
        })
 
        vals['task_id'] = task.id
        message = super(MyModel, self).create(vals)
        return message

In this example, when a new message is created, a new task is created with the subject of the message and the author of the message as the assigned user. The task is linked to the message using the task_id field.

Regards