You want
If you have a user token and want to look up that ID for that token
Use the users oAuth token
https://api.twitch.tv/helix/user
That’ll get the user data for that token
If you don’t have a user token
Use the any oAuth token
https://api.twitch.tv/helix/users?login=barrycarlyon
That’ll get the user data for barrycarlyon