Personally I would use the sticker extension, and stick the window to the header. Make sure you only have the drag behaviour on the header, not the window. Then when you drag the header, the window will move with it without you having to do anything. No need to fuss with IDs or placing one object based on another object.
Install the Sticker extension, and when you create the window and header, use the use Stick window to header (not the other way round).