That should be a URL and parameters, not headers.
https://id.twitch.tv/oauth2/token?client_id=TwitchService.clientID&client_secret=TwitchService.clientSecret&grant_type=client_credentials&scope=viewing_activity_read
That should be a URL and parameters, not headers.
https://id.twitch.tv/oauth2/token?client_id=TwitchService.clientID&client_secret=TwitchService.clientSecret&grant_type=client_credentials&scope=viewing_activity_read