For RSS/Atom, you put this in the XML, right inside the document element (the <feed> element or the <rss> element):

    <script src="https://example.org/script.js"
       xmlns="http://www.w3.org/1999/xhtml"></script>

You can also put CSS in there, like this:

    <style xmlns="http://www.w3.org/1999/xhtml">
      * { color: red; }
    </style>

Or like this:

    <link href="https://example.org/style.css"
       rel="stylesheet" xmlns="http://www.w3.org/1999/xhtml"/>