beforeCleanWord example

I need to adjust how ckeditor cleans content pasted from word. Looking through the code, I found the following:

plugins/pastefromword/filter/default.js

// Allow extending data filter rules.
editor.fire( 'beforeCleanWord', { filter : dataFilter } );

I believe I extend it by writing:

CKEDITOR.on( 'beforeCleanWord', function() {
    alert('in beforeCleanWord');
    return {
        liWithMargin : function(element)
        {
            alert(element);
        }
    };
});

Where do I put this code? I tried placing it in my custom config.js, but it doesn't appear to get called.

Thanks