old_contents: # -*- coding: utf-8 -*-
  from odoo import api, models
  
  class Website(models.Model):
      _inherit = 'website'
  
      @api.multi
      def _prepare_sale_order_values(self, partner, pricelist):
          self.ensure_one()
          values = super(Website, self)._prepare_sale_order_values(partner, pricelist)
          if values['company_id']:
              warehouses = self.env['stock.warehouse'].sudo().search([('company_id', '=', values['company_id'])], limit=1)
              if warehouses:
                  values['warehouse_id'] = warehouses.id
          return values
  
new_contents: # -*- coding: utf-8 -*-
  from odoo import api, models
  
  class Website(models.Model):
      _inherit = 'website'
  
      @api.multi
      def _prepare_sale_order_values(self, partner, pricelist):
          self.ensure_one()
          values = super(Website, self)._prepare_sale_order_values(partner, pricelist)
          if values['company_id']:
              warehouse_id = (
                  self.env['ir.values'].get_default('sale.order', 'warehouse_id', company_id=values.get('company_id')) or
                  self.env['ir.values'].get_default('sale.order', 'warehouse_id') or
                  self.env['stock.warehouse'].sudo().search([('company_id', '=', values['company_id'])], limit=1).id
              )
              if warehouse_id:
                  values['warehouse_id'] = warehouse_id
          return values
  
subject: use sale order warehouse default first