You can do it manually using this link
Code:
https://api.twitch.tv/kraken/oauth2/authorize?response_type=token&client_id=[your client ID]&redirect_uri=http://localhost&scope=channel_commercial

Just replace [your client ID] with your client ID.
Then you will be directed to error page which is fine.
If you do it correctly, you will see your access token at the url/address bar.
Copy and save it in your file for future reference.