While a common Javascript applies across all KB subsites, the Site Javascripts screen allows each KB site to apply/remove Javascript to their internal and/or external sites.
The JavaScripts link can be found in the left navigation bar in the SitePref tab.
The image below shows the JavaScripts link highlighted with a red box.
Once you click the JavaScripts link, you will be led to the Site Javascripts screen where you will see three text entry fields:
Please know that when you are creating/ modifying your script, opening and closing script tags (<script>...</script>
) do not need to be included as the KB system automatically inserts these tags when the javascripts are loaded.