I am trying to add a new field in pos order line and update the value in button click.

**pos.oerder.line.py**

    class pos_order_line(models.Model):

        _inherit = 'pos.order.line'

        is_promo = fields.Boolean(string='Promo',default=False)

***.js**

    models.Orderline = models.Orderline.extend({

    initialize: function() {

      _super_orderline.initialize.apply(this,arguments);

      this.is_promo = false;

    },

    // Help me to understand this function

    export_as_JSON: function(){ 

        var json = _super_orderline.export_as_JSON.apply(this,arguments);

        json.is_promo = this.is_promo;

        return json;

    },

    // Help me to understand this function

    init_from_JSON: function(json){

        _super_orderline.init_from_JSON.apply(this,arguments);

        this.is_promo = json.is_promo;

    },

    });

    var OfferButton = screens.ActionButtonWidget.extend({

    'template': 'PromotioanlButton',

    button_click: function(){

        var self = this;

        self.get_promo_config();

    },

    get_promo_config: function () {

      var self = this;

      var order    = this.pos.get_order();

      var orderline = order.get_orderlines();

      var order_product_ids = []

      _.each(orderline,function(line){

        order_product_ids.push({'product_id':line.product.id,'qty':line.quantity});

      });

      rpc.query({

        model:'promo.config',

        method:'check_promo_offer',

        args :[order_product_ids],

      }).then(function(product_ids){

        _.each(product_ids, function(pid){

          var product  = self.pos.db.get_product_by_id(pid.product_id);

     **// I added new item into cart using below code and its working fine. 

     //It add all the fields below mention except `is_promo`, i mean it still false. 

      //how can i make it true.**

          order.add_product(product, { price:0.0 ,quantity:pid.product_uom_qty,discount:25,is_promo:true,merge:false});

      });

      });

    },

    });

Please suggest any solution.