How can i create a note on my webpage for iphone user that there is an app?

How can i create a note on my webpage for iphone user that there is an app?

i startet to detect which platform is be used. And this already works:
// User-Agent-String auslesen
var UserAgent = navigator.userAgent.toLowerCase();
// User-Agent auf gewisse Schlüsselwörter prüfen
if (UserAgent.search(/(iphone|ipod|opera
mini|fennec|palm|blackberry|android|symbian|series60)/) > -1) {
// mobiles Endgerät
alert("YOU HAVE A SMARTPHONE");
} else {
// kein mobiles Endgerät (PC, Tablet, etc.)
alert("YOU HAVE NO SMARTPHONE");
}
but now I want to show on the iphone IF you have an iphone a full screen
page that there is an app and you should download it. and than a button
for the app and a button for "no thank you, continue to normal page".
Can someone help me? The page should be full screen and nothing else
should be visible. thank you