First create the inline labels following the instructions here: http://documentation.unbounce.com/hc/en-us/articles/203799174-Adding-and-Editing-Form-Fields/#content7
Next, paste the following code in the Javascripts section, selecting 'Head' as the placement:
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.3/jquery.min.js"></script>
<script src="https://cdn.rawgit.com/derpoho/floatlabels.js/master/floatlabels.js"></script>
<script>
$(document).ready(function () {