Youtube API request credentials

I created an python application that is using the Youtube api (so examples are in python, but doesn't really matter, the concepts should be the same). I managed to get it working where I can connect and make api calls. However, when I connect to the api, I have to define a flow that checks if a the credentials storage file exists. If it doesn't, then I have to manually sign in using the flow. After sign in the file (main.py-oauth2.json), is created with the token. I would like to be able to download the credentials without having to sign manually sign in. I was hoping there was a way to make a POST request for that token, like I have seen here, but I have been able to do this with Youtube api. Does anyone know how to implement the desired feature ?

main.py

flow = flow_from_clientsecrets(CLIENT_SECRETS_FILE,
    scope=YOUTUBE_UPLOAD_SCOPE,
    message=MISSING_CLIENT_SECRETS_MESSAGE)
storage = Storage(OAUTH_CREDENTIALS)

credentials = storage.get()

if credentials is None or credentials.invalid:
    # manual / UI login
    credentials = run_flow(flow, storage, args)

youtube api Oauth flow docs

from Youtube API request credentials