Look at using a class name instead of an id.
Also you may need to then make sure you have the right one on the current scene element.
There are plenty of example on site in getting the current scene element including using the
Extension hypeDocument.sceneInfo
in the
You can then querySelector() that scene element for the class
i.e
var sceneInfo = hypeDocument.sceneInfo()
var child = sceneInfo.sceneElement.querySelector('.header');