Users’s don’t have a client ID. A client ID represents an application.
You should register your application Authentication | Twitch Developers and then using the credentials for that application you can have users go through your apps OAuth flow, or create App Access Tokens for any API endpoint that doesn’t require user scopes.
That Client ID for your app you’ll use for ALL of your requests, regardless of which user you’re requesting data for.
If you want to get a user id, you can use the Get Users endpoint Reference | Twitch Developers with any OAuth token, be it a User Token, or an App Token, as that endpoint doesn’t require user scopes (unless you need a users email, in which case you do need a user token from that user).