I am create module cal salesman

1. i created model

salesman.py

class Salesman(models.Model):
 _name = 'smartsales.salesman'
 _description ='Salesman'
 _inherit = 'mail.thread'
salesname = fields.Many2one('res.users', string='Name', defalut=lambda self: self.env.uid)
companyname = fields.Char(string='Companyname')
@api.onchange('salesname.company_id','companyname')
 def update_company(self):
  self.companyname = self.salesname.company_id

view

<field name="salesname"  />
<field name="companyname" on_change="0" readonly="1"/>

I want to create dropdownlist that when i select user and it will show companyname  of user 

how can i do it ?