Okay, got this figured out
Thanks for the links
Got successfully subscribed with the app to the pubsub topic for channel point redemptions 
- Open https://id.twitch.tv/oauth2/authorize?client_id=$client_id&redirect_uri=http://localhost&response_type=code&scope=channel:read:redemptions%20chat:read in browser
- Get the
?code=from the URL - Paste the code to and
curl -X POSThttps://id.twitch.tv/oauth2/token?client_id=$client_id&client_secret=$client_secret&code=$code&grant_type=authorization_code&redirect_uri=http://localhost - Use the
access_tokenas header “Authorization: OAuth $access_token” oatn https://id.twitch.tv/oauth2/validate - ???
- Profit!
Everytime I was so confused on the redirect_uri that I totally ignored what was in the parameters when I opened the link in step 1.
What also helped understanding was this blog post: https://blog.twitch.tv/en/2019/11/06/twitch-authentication-understanding-which-protocol-and-flow-is-right-for-you/
Issue resolved for me 