Marmaris
Not checked, but this could be used to detect instagram browser:
<script>
function isInstagram() {
var ua = navigator.userAgent || navigator.vendor || window.opera;
return (ua.indexOf("Instagram") > -1);
}
document.addEventListener("DOMContentLoaded", function(event) {
if (isInstagram()) {
alert("Enter your custom message here...")
}
});
</script>