Warning message for browser exit

Hi all, not sure this is something that can be done in Storyline 360 but I thought I would ask in case someone has figured out a way to do this.

I have a Storyline module at the end of weekly lessons that ask a series of MC and short answer questions.  The last page (results page) triggers the LMS passback "complete" status through a button on the final page to "Exit Reflection".  That button also closes the window.  I have students that either close the browser window (X) before reaching the results page or when on the results page, use the browser close (X) instead of my "Exit Reflection" button.  Either way, no complete flag is sent and no score added in their Blackboard grade book.   

Anyone know of a way if the student selects the browser close (X) button, that it pops up a warning message "Are you sure you want to close, your grade will not be submitted (Y/N)"?

Thanks, Eric