Hi @Pascal Tremblay

Here is how:

openerp.pos_test = function (instance) {
    var _t = instance.web._t,
    _lt = instance.web._lt;
    var QWeb = instance.web.qweb;

    instance.point_of_sale.PaymentScreenWidget.include({
        init: function(parent, options) {
            var self = this;
            this._super(parent,options);
            document.body.removeEventListener('keyup',this.hotkey_handler);
            this.hotkey_handler = function(event){
                if(event.which === 13){
                    self.validate_order({invoice: true});
                }else if(event.which === 27){
                    self.back();
                }
            };
            document.body.addEventListener('keyup', this.hotkey_handler);
        }
    });
}