Hi,

You can load XML from  __manifest__.py file

'qweb': ['static/src/xml/*.xml'],

then, custom_pos.xml

<?xml version="1.0" encoding="utf-8"?>
<templates id="template" xml:space="preserve">
    <t t-extend="PaymentScreenWidget">
        <t t-jquery="t[t-if='widget.pos.config.module_account']" t-operation="after">
            <div class='button js_custom_print'>
                <i class='fa fa-print' /> Custom Print
            </div>
        </t>
    </t>
</templates>

load JS :-

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data>
        <template id="assets" inherit_id="point_of_sale.assets">
            <xpath expr="." position="inside">
                <script type="text/javascript" src="/POS/static/src/js/pos_test.js"/>
            </xpath>
        </template>
    </data>
</odoo>

JS File:

odoo.define('POS.pos', function (require) {
'use strict';

var models = require('point_of_sale.models');
var core = require('web.core');
var screens = require('point_of_sale.screens');
var Widget = require('web.Widget');

var ajax = require('web.ajax');

var qweb = core.qweb;

screens.PaymentScreenWidget.include({
    renderElement: function() {
        var self = this;
        this._super();
        this.$('.js_custom_print').click(function(){
            self.click_custom_print();
        });
    },
    click_custom_print: function(){
        var order = this.pos.get_order();
        console.log("order")
        // Render receipt screen and can print function

    }
});

});

Thanks