You could try using the client side event AfterItemSelected for such scenario. Here is the snippet:

function cmb_OnAfterItemSelected(controlId)
{
	var cmb = ISGetObject(controlId);

	var msg = "Selected item \n" + 
	    "Value: " + cmb.Value + "\n" +
	    "Text: " + cmb.Text + "\n" +
	    "Index: " + cmb.SelectedIndex;
	
	alert(msg);
	
	return true;
}

this event will be fired after an item is selected. not when data is loaded. so the client will have to select one row. which is not what I require. is there any event which is fired when the data is loaded?

or can i do the following:

add a row in the client side code.

use the onrowadded event.

will this row get added after the data is populated on the result box from data source?

I suggest you to try OnShowDropDown client side event. OnShowDropDown client side event specifies the client side (Javascript) function that will be invoked when the ResultBox is about to show.

Hope this helps. Please let me know if you have different scenario.