Answer: It is not possible to use Service accounts with client sided JavaScript. It is not supported by Google’s Oauth system.

Reason

This is due to the security ramifications. Weather or not you feel this is a security risk is moot. Its not supported because it is a security risk.

You will need to switch to a server sided programming language if you want to use service account authorization.

If you do decided to switch languages, remember to configure domain wide delegation to your google workspace domain and remember to include the user for which you want to delegate.

Note. Service accounts only work with Google calendars though workspace domains not standard Gmail domains.