Totally missed that box when reading since the example/generates code mentions v3 when you should be using v5 and that header doesn’t seem to have a entry in the custom headers for it.
Edit: In fact he’s getting a clientID error at the wrong step so the custom headers don’t apply as you pass the clientID in the first step as a query string argument not a header. So
@Ross_Di When you are getting this error before or after the redirect and return to twitch?
@Anaerin if you pass an oAuth token that is valid, you don’t need a ClientID. As Twitch works out the client ID from the oAuth.