Thank you for the reply.

I am using QQuickWindow and i am not using QWidget.
I have created a custom window i.e "class CustomWindow : public QQuickWindow {...}"
This CustomWindow only i have used in QML side. and sorry in my first thread i have mentioned ApplicationWindow
So only I am not able to access or pass QQuickWindow to QWidget::stackUnder(QWidget *w).
Requesting you to please let me know if there is any other solution.