Hi,

The create _prepare_invoice in sale order creates the corresponding invoice. For passing any value from the sale order to the creating invoice inherit sale.order model and override the function. 

Try this example code

def _prepare_invoice(self):
   invoice_vals= super(SaleOrder, self)._prepare_invoice()
   invoice_vals['invoice_field_name'] = self.client_order_ref
   return invoice_vals

Regards