You can't place Gamestudio controls into this subwindow.
I thought this whole thread was about how to add Windows elements like ComboBoxes to a Gamestudio application. It is not possible in the main window because DirectX would render above it, so I showed you how to do it by adding a subwindow. The whole point of adding this subwindow was creating an area in which the engine doesn't render, so you can create Windows elements into it. This means, in the subwindow, you can only create Windows controls, while in the main window you can only create engine controls.