I could not find an authorized Spotify document that supports your assumption (this is the best I could find: 358170).
I would nevertheless gladly add your key to the addon, if you provide it to me.
You can probably use it but you need to keep it in the encoded source code instead of some config file because people can copy the app key and use it in their own application. When their application violates spotify rules the owner of the key will get a fine/ban.
So it is possible but you need to hide it in a non open-source/encoded file.