You may include the following kinds of HTML content in your app's HTML Header:
<script> .. </script> tags, see:  w3Schools <style> .. </style> tags, see:  w3Schools <link> .. </link> tags, see:  w3Schools <meta> .. </meta> tags, see:  w3Schools Script tags may include external libraries and any custom Javascript or jQuery. Keep in mind that JavaScript code in a custom header will not execute until some action calls your code. You can find out more about adding custom JavaScript and jQuery code in these sections:
All JavaScript code that you include in the HTML Header must be encased within 
<script> .. </script> tags.