I don’t think you can do that. What you have to do is put your hbox as a child of a margin_container. Then add your texture rect as another child of the margin container. The margin container will expand to the size of the hbox, and then you can stretch your texture rect or tile it etc. The nine patch rect works superbly well in this situation if your texture needs it. A panel works very well too in this situation.
Hope that helps in some way.