Use QWidget derived class in Dialog

Hello, i have a problem:

i have created a dialog in qt designer and i want a QWidget derived class to be shown there in a box, how can i do that?

Place a QWidget on the form and promote it as your custom widget.

thanks, that works great. you really saved my day, have been trying for hours and now its all ok with a few clicks and like 3 lines of code :)