Set up the widget as follows:
Parent it to your main widget
Anchor it to 0.5 0.5
Set Alignment to 0.5 0.5
Set position to 0 0
Anchor 0.5 0.5 will make the center of the coordinate system the center of the screen.
Alignment to 0.5 0.5 will make the center of the widget the center of itself, therefore positioning the widget center of the widget in the center of the screen.
Position 0 0 will then translate it by nothing, leaving your widget in the center of the screen.