Lookup Control PreSearch Event (Client API reference)
This event occurs just before the Lookup control launches a dialog to search for records. There is no UI to set event handlers for this event. You must use the addPreSearch and removePreSearch methods on the lookup control to add or remove event handlers for this event.
Use this event with other Lookup control methods to change the results displayed in a lookup based on the form data just before the lookup control shows search results for a user to choose from.