jQuery Simple Example

HTML

<script src="//www.addressfinder.co.nz/assets/v2/widget.js"></script>
<textarea placeholder="Enter some text..."></textarea>

CSS

body {
  margin: 50px;
}

textarea {
  font-family: "Arial";
  width: 600px;
  height: auto;
  padding: 16px 20px;
  line-height: 20px;
}

JavaScript

$(document).on('input', 'textarea', function(event) {
  var el = $(this);

  if (el.val()) {
    el.addClass('active');
  } else {
    el.removeClass('active');
  }
});