Hi, 

Please refer the following code to add custom field in POS.

in .js

odoo.define('module_name.custom_field', function(require) {
    "use strict";
     var models = require('point_of_sale.models');
      var product_barcode = models.load_fields("product.product", 'field_name');
      }); 

in .xml

in the below xml we are adding the loaded field into product view

<templates id="template" xml:space="preserve"> <t t-name="ProductItem" t-inherit="point_of_sale.ProductItem" t-inherit-mode="extension" owl="1"> <xpath expr="//div[hasclass('product-img')]" position="after"> <t t-if="props.product.custom_field"> <span style="font-size: 10px;" class="price-tag" t-esc="props.product.custom_field"/> </t> </xpath> </t> </templates>

Regards