InfoPath Dev

In the book 'Professional InfoPath 2003' it mentions that you can use the 'OnLoad' event handler to create a secondary data source, eg. XML for populating a drop-down list. I wnat to use this so the XML filename is not hard-coded. How do you set the properties under the drop-down control/secondary data source to work with this as I've tried various combinations and the data source specified always overrides the OnLoad one ?! I've only recently started looking at using InfoPath so apologies for any confusion here.

I think question I'm after is: what value do you set the 'Data Connection' to in the 'Drop-Down List Properties' box when overriding using an eventhandler ?

Forget this please as I've managed to get it working, I had attached the data source to the form by accident (embedded it in the template) which was overriding my changes. The default file name specified in the data source within the drop-down list properties is indeed overridden by the event handler.