1. Go to Content > Content Management System
2. Hover over the name of the CMS page and click Manage Website
3. Click the Administer tab
4. Click the Add-Ons tab
5. Click Edit and add your code
6. Save your edits

Note:
If you don't see the Add-Ons tab, go to: Administer > Privileges and click the Staff Privileges link.
Put a check in the Add-ons Column for all users that need to see the tab