Sencha Architect create override

I want to create an override for the Ext.MessageBox. I have created the override file manually and pointed a JS resource to it. Now Architect sees this file as an override. But when calling the override function I get an error. If I look throught the Ext.MessageBox object the new function is not there. How can I specify overrides in Architect? What excactly am I doing Wrong? /app/overrides/MessageBox.js

Code:

 Ext.define('myapp.override.MessageBox', {     override: 'Ext.MessageBox',      test: function(){         alert("123"); }  });

calling: Ext.MessageBox.test(); gives: Uncaught TypeError: Object [object Object] has no method 'test'