Try this:
Code:
YESNOPopup.createInstance("Headline", "Content", new IYesNoListener() {
@Override
public void reactOnYes() {
//Yes
}
@Override
public void reactOnNo() {
//No
}
}).getModalPopup().setLeftTopReferenceCentered(); //centered
or for OKPopups:
Code:
OKPopup.createInstance("Headline", "OK?", () -> System.out.println("OK")).getModalPopup().setLeftTopReferenceCentered(); //centered