Follow these steps to add the 5% discount action button on the keyboard panel of the Point of Sale application:

1. Add the following code to the /static/src/js/pos_demo.js file, which will define the action button:

odoo.define('pos_demo.custom', function (require) {
 "use strict";

 var screens = require('point_of_sale.screens');
 var discount_button = screens.ActionButtonWidget.extend({
     template: 'BtnDiscount',
     button_click: function () {
         var order = this.pos.get_order();
         if (order.selected_orderline) {
             order.selected_orderline.set_discount(5);
         }
     }
 });

 screens.define_action_button({
     'name': 'discount_btn',
     'widget': discount_button
});

});

2. Add the QWeb template for the button in the /static/src/xml/pos_demo.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<templates id="template" xml:space="preserve">

    <t t-name="BtnDiscount">
        <div class='control-button'>
            <i class='fa fa-gift' /> 5% discount
        </div>
    </t>

</templates>

3. Register the QWeb template in the manifest file as follows:

'qweb': [
        'static/src/xml/pos_demo.xml'
     ]

Update the pos_demo module to apply the changes. After that, you will be able to see a 5% discount button above the keyboard:

After clicking this, the discount will be applied on the selected order line.