happening validate event after Selection list is selected and closed

Hi

I have a key field that "ItemCode" in my form.

when the field is empty and tab key is pressed, then selection list is opened. I select the item from list and press OK, then item is filled in ItemCode field.

In this case, no event is happen. so i must press the tab key or click another fields to make running validate event. but i wanna make it directly happen when field is filled from selection list.

If you have a solution or sample code about it, please help me.