odoo.define('uni_pos.ProductScreen_PosLineDiscount', function(require) {
   'use strict';
   const Registries = require('point_of_sale.Registries');
   const ProductScreen = require('point_of_sale.ProductScreen');

    const PosLineDiscount = (ProductScreen) => class extends ProductScreen {

        async _clickProduct(event) {
            if (!this.currentOrder) {
                this.env.pos.add_new_order();
            }
            const product = event.detail;
            const options = await this._getAddProductOptions(product);
            // Do not add product if options is undefined.
            if (!options) return;
            // Add the product after having the extra information.
            this.currentOrder.add_product(product, options);
//            this.currentOrder.get_selected_orderline().set_discount(5)
            console.log('Product Add')

            try {
              NumberBuffer.reset();
            }
            catch(err) {
              console.log(err)
            }
        }
    };
    Registries.Component.extend(ProductScreen, PosLineDiscount);
    return ProductScreen;

});

Here is the answer, for extending a OWL class and adding and overriding a function.