API Authentication in JS

API
9

Thank you very much!

var XML = new XMLHttpRequest();
            
            XML.open("GET", "https://api.twitch.tv/helix/streams/?user_id=sugarincubes");
            XML.setRequestHeader('Client-ID', '<clientID>');
            XML.send();
            XML.onload = function () {
              console.log(XML.response);
            }

Gave me some meaningful output at last.
However, I only ever get a JSON object with data: [] regardles of the stream being online of not. Does the user_id differ from the name of a channel?