I believe the issue is that there is no field on the userpool GUI to add custom parameters.
Where are claims made and sent on OpenID and what type of response object should I find from Twitch?
It’s been a while since I’ve tried using Cognito with Twitch, I gave up because at the time there was no support for custom claims at all, meaning the only claims you receive are the default ones that Twitch sends.
It is possible to get it to work, as I did successfully set it up, but without being able to request any additional claims I found that it wasn’t really fit for my purpose and gave up on the service in favour of just doing the auth process myself with other AWS services.