Extension Helper Library
An extension’s iframe must import the Extension Helper JavaScript file, created and hosted by Twitch. It provides methods for dealing with authentication, receiving notifications of stream properties, and listening to PubSub events. Each HTML defined in the Asset Hosting section of your extension (Viewer, Config, Live Config) must load the Extension Helper. To do so, include this line:
<script src="https://extension-files.twitch.tv/helper/v1/twitch-ext.min.js"></script>
You could load jQuery from code.jquery.com for example. That is jQuery’s CDN. You cannot use a CDN.
Manage an extension and look under, Versions, A Version, Access
You can enter Testing Accounts as TwitchID’s here