You need to go through the Authentication process, documented here: https://dev.twitch.tv/docs/authentication, using either the Implicit or Auth Code flow with the channel:read:redemptions scope so that you can grant a User Token.
Keep in mind that you can only access the redemptions for rewards created by the Client ID you’re using (so you wont get redemptions for rewards created on the dashboard), and only for the user that goes through that OAuth process to grant you those permissions.