Find out if stream is live

We try to call
https://api.twitch.tv/extensions/[extension ID]/live_activated_channels

But if I add for [extension ID] the 6 digit Extension ID which is displayed in developer dashboard, I get response 400 - Bad request “No client id specified”

If I additionally add the client-ID in the request Header, I get 404 - Not Found “The record was not found”

If I call
https://api.twitch.tv/extensions/[client-ID]/live_activated_channels
(that is shown in the example of documentation) I get the same 404 error. Independent if I put the client-ID additionally in the header or not.

What I do wrong?