Hi @loay elhaj

let suppose you have created the field employee_ids in your cutom model as test.
employee_ids = fields.Many2many('hr.employee', 'employee_test_rel', 'test_id', 'emp_id', string='Employees')

to get the ids of the selected employees in many2many field, you have to do
self.employee_ids.ids
this will return a list of selected employee ids ([6,889,90] for example)