The <script> tag is always removed from the HTML that is inserted in the editor. I need to keep it in order to add a Google tracking script into an email template.
The editor is removing by default the <script> tag in order to prevent possible XSS attacks. Although we don't recommend to do that, you can allow it inside the editable area by changing the htmlRemoveTags option.
The best approach for setting the Google Tracking code would be to create a custom button that would show a popup asking for the tracking ID (UA-000000-01) and storing it separately. Then, you could use it to inject the Google Analytics script in the HTML output of the editor. This way you would be safe from XSS injection and it would be easier for your users to insert the script.