Authentication

It is not possible to fully automate the OAuth process where the user chooses to authorize the app and the app then receives the resulting access token and optional refresh token. This needs to be done manually by the user at least once.

If your app needs to maintain long-term access without the user manually re-authorizing it repeatedly, the app should request "offline" access so that it gets a refresh token. The refresh token doesn't expire and can be stored and used repeatedly to get new short-lived access tokens whenever needed, without the user manually reauthorizing the app.
 
You can find more information in the OAuth Guide and authorization documentation.