QPushButton Main Gui

QPushButton Main Gui

I'm beginning with Qt and I do not understand something. Here is my code:
#include <QApplication>
#include <QtWebKitWidgets/QWebView>
#include <QMainWindow>
#include <QPushButton>
QApplication a(argc, argv);
QMainWindow *mWin = new QMainWindow();
QPushButton *button = new QPushButton("Button");
button->setGeometry(0,0,128,56);
button->show();
mWin->setGeometry(QRect(300,300,1024,640));
mWin->setWindowFlags(Qt::CustomizeWindowHint |
Qt::WindowMinimizeButtonHint | Qt::WindowCloseButtonHint);
mWin->show();
return a.exec();
When I launch the program, it makes a pushbutton, but not on the main GUI.
It instead makes it in the upper lefthand corner of my scoeen. How do I
make the pushbutton part of the main GUI?