Lookup to the same form?

Here's what I need to do... I have a form with "activities" that needs to allow the user to select a "parent" activity from a dropdown. The parent dropdown should be populated by previous entries from the activites form or null if no other entries have been made. Zoho won't allow me to create a lookup to the same form, so I tried a little code for the form "On Add", but it doesn't seem to work...

Any ideas on how to lookup previous entries to the same form?