Getting the User Agent with JavaScript
Pure Javascript
document.getElementById('UserAgent').value = navigator.userAgent;<input type="text" id="UserAgent">jQuery
$('#UserAgent').val(navigator.userAgent);<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script><input type="text" id="UserAgent">
Original Q didn't say anything about jQuery. so
document.getElementById('UserAgent').value = navigator.userAgent;