I've read only now this:

 * This file was generated by Ext Designer version 1.0.0.
 * http://www.extjs.com/products/designer/
 * This file will be generated the first time you export.
 * You should implement event handling and custom methods in this
 * class. 

ChiamateViewport = Ext.extend(ChiamateViewportUi, {
    initComponent: function() {
Could you tell me the way to do this?

In my element ChiamateViewport I've all my sub elements, one is my combobox and I want to "attach" listeners and onLoad actions...

This is the generated script in the .ui.js file:
    xtype: 'combo',
    fieldLabel: 'Chiamante',
    anchor: '100%',
    tabIndex: 1,
    allowBlank: false,
    loadingText: false,
    mode: 'remote',
    store: 'store-chiamanti',
    emptyText: 'Cerca Chiamante...',
    selectOnFocus: true,
    id: 'ricerca-chiamante'