void MainWindow::createCommands()
{
    // ...
    this->addCommand<MyCommandCreateBox>();
}

void MainWindow::createMenus()
{
    // ...
    {   // Tools
        auto menu = m_ui->menu_Tools;
        fnAddAction(menu, CommandSaveViewImage::Name);
        fnAddAction(menu, CommandInspectXde::Name);
        menu->addSeparator();
        fnAddAction(menu, CommandEditOptions::Name);
        menu->addSeparator();
        fnAddAction(menu, MyCommandCreateBox::Name); // <-- YOUR COMMAND ACTION INSERTED HERE
    }

void MyCommandCreateBox::execute()
{
    auto guiDoc = this->context()->currentGuiDocument();
    auto doc = guiDoc ? guiDoc->document() : nullptr;
    if (doc) {
        // Create box BREP shape
        const TopoDS_Shape shapeBox = BRepPrimAPI_MakeBox(100, 100, 100).Solid();
        // Declare a label in the XDE structure to store the created shape
        const TDF_Label labelShape = doc->xcaf().shapeTool()->NewShape();
        doc->xcaf().shapeTool()->SetShape(labelShape, shapeBox);
        // Set a name associated to the shape
        TDataStd_Name::Set(labelShape, "MyBox");
        // Add root entity to the document, throws signal Document::signalEntityAdded caught by GuiApplication that
        // will automatically create/add 3D graphics in the corresponding GuiDocument object
        doc->addEntityTreeNode(labelShape); 
    }
}