I want click button create sale order data product in form custom transfer to form sale order.

file.py:

class custommodel(models.Model):
    _name = 'custom.test'    

    _inherit = ['sale.order']     

def create_sale_order(self):           

 return {            

           'name': 'Create Sale Order',          

           'res_model': 'sale.order',           

           'type': 'ir.actions.act_window',              

            'view_type': 'form',             

            'view_mode': 'form',            

            'views': [(self.env.ref('sale.view_order_form').id, 'form')],             

             'view_id': self.env.ref('sale.view_order_form').id,          

              'target': 'current',                }   

class citem(models.Model): 

     _name = 'item'

product = fields.Many2one('product.product', string='Product Name') 

      c_id = fields.Many2one(' custom.test')        

file.xml:

                    <button name="create_sale_order" string="Sale Order"  type="object" class="oe_highlight"  

context="{'default_product_id': product}"/>

Error:   name 'product' is not defined​ 

What to do to make the product field known in class   custom.test , transfer to sale.order.line؟​