where and how to display a warning message (app.alert) when press ctrl+p or use file>print?

Hi,

I want to display a warning message in pdf form when user press ctrl+p or use file>print. I use this code 

if(event.ctrlKey && event.keyCode == 80) {
app.alert("You pressed Ctrl+P");
}

in document javascript and in a text field but it does not work. I have created a print button in pdf which display the message (if and only if you use button for printing). I want the same message to appear when use different printing option.

Thanks