How can i add floating component while creating viewport in ext4.1

How can i add a floating window to the viewport while its getting created. When i'm trying to add with viewport.on('add',function(){//my functionality here});it's not working..How can i add??? Any help??

2 options
1/. Define in the viewport items config the window
See here for adding items to viewport - http://docs.sencha.com/ext-js/4-1/#!/api/Ext.container.Viewport

2/.add the window to the created viewport afterwards.

Code:

---

var window = Ext.create('Ext.window.Window', {
    title: 'My Window',
    height: 200,
    width: 400
});
window.show();
viewport.add(window);

---

@Suzuki
Thanks for your response