The kraken/streams/<user_id> endpoint uses a user_id (the same as channel_id) where you have provided their login (user name).
If you donβt already have the user_id, you can get it by either using the kraken/users endpoint or the helix/users endpoint. Both of these endpoints can look up a user based on their login via query parameters which will return a user object that contains their user_id.
Just keep in mind that the Kraken API is being removed completely at the end of the year, and that the same functionality can be obtained by solely using the Helix API.