<li>
  <label for="first-name">Your code:</label>
  <input type="text" name="code" id="short_link"  style="border: 1px solid #3E7EFF; border-radius: 5px 5px 5px 5px;">
</li>
<li>
  <div id="gen" style="cursor: pointer";>Generate</div>
</li>

$(function() {
    function str_rand() {
        var result = '';
        var words = '0123456789qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM';
        var max_position = words.length - 1;
        for (i = 0; i < 15; ++i ) {
            position = Math.floor ( Math.random() * max_position );
            result = result + words.substring(position, position + 1);
        }
        return result;
    }

    $("#gen").click(function() {
        $("#short_link").val(str_rand());
    });
});