Greetings everyone i'm trying to do develop on this screen ( Search Form )

i need to run notification sound or message when this form appear
after cashier check barcode from this screen

and if not found it show me search form
i did try by ItemTriggers but still can't reach to that form (search form)

if i make put like this
if (saleLineItem.Found)
{
MessageBox.Show("Item found");
SystemSounds.Question.Play();
}
and i put barcode for item exist that's message appear
but i need message appear for item doesn't exist before ( Search Form ) show