I need to apply a discount depending on the payment form used. Since seems a bit complicated to do, I've opted for adding a global discount on payment screen, replicating a function already present (pos_discount module).

 JS code:

odoo.define('pos_discount.pos_discount2', function (require) {
"use strict";

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

var _t = core._t;

var DiscountButton = screens.PaymentScreenWidget.extend({
    template: 'DiscountButton2',
    button_click: function(){
        var self = this;
        console.log (this);
        this.gui.show_popup('number',{
            'title': _t('Discount Percentage'),
            'value': this.pos.config.discount_pc,
            'confirm': function(val) {
                val = Math.round(Math.max(0,Math.min(100,val)));
                self.apply_discount(val);
            },
        });
    },
    apply_discount: function(pc) {
        var order    = this.pos.get_order();
        var lines    = order.get_orderlines();
        var product  = this.pos.db.get_product_by_id(this.pos.config.discount_product_id[0]);

        // Remove existing discounts
        var i = 0;
        while ( i < lines.length ) {
            if (lines[i].get_product() === product) {
                order.remove_orderline(lines[i]);
            } else {
                i++;
            }
        }

        // Add discount
        var discount = - pc / 100.0 * order.get_total_with_tax();

        if( discount < 0 ){
            order.add_product(product, { price: discount });
        }
    },
});

screens.define_action_button({
    'name': 'discount2',
    'widget': DiscountButton2,
    'condition': function(){
        return this.pos.config.iface_discount && this.pos.config.discount_product_id;
    },
});


});

XML:

<?xml version="1.0" encoding="UTF-8"?>
<templates id="template" inherit_id="point_of_sale.template">

    <t t-extend="PaymentScreenWidget">
        <t t-jquery="div[class='payment-buttons']" t-operation="after">
            <t t-name="DiscountButton2">
                <div class='control-button js_discount'>
                    <i class='fa fa-tag' /> Discount
                </div>
            </t>
        </t>
    </t>

</templates>

Template for inheriting:

<odoo>    
    <data>
        <template id="assets" name="pos_discount2_data assets" inherit_id="point_of_sale.assets">
          <xpath expr="." position="inside">
                <script type="text/javascript" src="/pos_divina_custom/static/src/js/cpf.js"></script>
            </xpath>
        </template>
    </data>
</odoo>

The button show right where I want, but I can't get a link to the event on mouse.click. Any clue?