That is not an incident form, that is a ticket conversation form. It has nothing to do with any Client Scripts.
You need to either modify the summary or conversation widgets to watch the current record and if it is an incident and the state changes to show the message, or create a new widget that watches the current record and does the messaging if the current ticket is an incident and is being closed. You'd than add the new widget to the page.
Creating a new widget will be easier I would say.
