I added field in pos.order.line named cashier_name which is a Char
and i am trying to set value to it when product choosed 

i added this code in js , but the field appears empty in pos order line

the value does not saved to field

odoo.define('pos_set_default_customer.get_customer', function (require) {
    var models = require('point_of_sale.models');
    var posModelSuper = models.PosModel.prototype;

var product_model = posModelSuper[product_index]
models.load_fields("pos.order.line", ["cashier_name"]);
    var order_model_super = models.Order.prototype;
    models.Order = models.Order.extend({
    add_product: async function (product, options) {
        var cashier_name=this.pos.order_id;
        var x= await order_model_super.add_product.apply(this, arguments);
        var selectedLine = this.get_orderlines().find(line => line.product === product);
        selectedLine.set("cashier_name","Test Value");

        return x;
        }, });});