I am currently developing a google hangout chatbot.

For that, I need to know, how to create an access token for the hangout Chat API - REST API(https://developers.google.com/chat/api/guides/rest-api) without creating manually.

I have tried the OAuth 2.0(https://developers.google.com/identity/protocols/oauth2) concepts which do not work.

Also, Need to know how to create a private key for the service account in code level using app script/java/javascript

Kindly advise on this.