How to make a Qt Widget grow with the window size?

Question

I want to have a small QFormLayout that grows to fill its parent widget.

I created a new .ui file using the QWidget template in Qt

Answer 1

I found it was impossible to assign a layout to the centralwidget until I had added at least one child beneath it. Then I could highlight the tiny icon with the red 'disabled' mark and then click on a layout in the Designer toolbar at top.