one2many (transientModel) on button click

one2many_transientModel_Exa.py

from odoo import models, fields                                                    
                                                                                                                                                                    
class DbconnectorMessageBox(models.TransientModel):                                
                                                                                   
    _name = 'dbconnector.messagebox'                                               
    _description = 'MessageBox Popup'                                              
    _rec_name = 'text'                                                             
                                                                                   
    def _default_engine(self):                                                     
                                                                                   
        engine_obj = self.env['dbconnector.engine']                                
        engine_id = self._context.get('active_id')                                 
        engine_record = engine_obj.browse(engine_id)                               
        return engine_record                                                       
                                                                                   
    engine_id = fields.Many2one("dbconnector.engine", string="Engine",             
                                required=True, default=_default_engine)            
    text = fields.Text()