How to catch field value after the form is filled?

Hello,

I added few comboboxes to the form "Item Card".

The selected values of the comboboxes must be depended on ItemCode.

So I need to get ItemCode when the item data is loading to the form (OK_MODE, UPDATE_MODE).

Which event (and how?) can I use to catch field value (in my case, item code) after the form is filled?

Thank you