Вот как вы можете это сделать:
QPushButton *btn = new QPushButton("Click Me", this); //this is parent, give another window/widget address to draw/create it there
btn->setGeometry(100, 200, 150, 50); //set position, width, height
connect(btn, SIGNAL(clicked()), this, SLOT(a_slot())); //make it useful
btn->show();
btn->setText("Push Me"); //change button text later if you want