Hello,
Your user_agent is a string of your choice that uniquely identifies your project. This can be useful for troubleshooting, since we would be able to look up your requests by the user agent.
The client_secret should have been generated along with your client_id, and you can look it up in the Google Developers Console.
The refresh_token is how you authenticate with OAuth 2.0. Please read our
guide on OAuth 2.0 for AdWords to get started on generating a refresh token.
Regards,
Mike, AdWords API Team