Multiple Selection in QComboBox

I have two windows as shown in the figure below (MainWindow, and Dialog). There is a button (Second Window) in the MainWindow by which I can switch to the "Dialog" window as well as create "QComboBox" through the "CheckableComboBox" class. The class code is supposed to provide a combo-box with multiple selections capability. It is highly appreciated if you let me know how to solve the following problems:

1. Is there a way to add the "checkbox" options on the left side of combo-box items?

2. How do I access to the combo-box items? Thank you so much in advance! enter image description here

            from PyQt5 import QtGui, QtCore, QtWidgets
            import sys, os
   
            from First import Ui_First_MainWindow
            from Second import Ui_Second_Dialog
   
   
            class CheckableComboBox(QtWidgets.QComboBox):                    
                def addItem(self, item):
                    super(CheckableComboBox, self).addItem(item)
                    item = self.model().item(self.count()-1,0)
                    item.setFlags(QtCore.Qt.ItemIsUserCheckable |QtCore.Qt.ItemIsEnabled)                
                    item.setCheckState(QtCore.Qt.Unchecked)
   
                def itemChecked(self, index):
                    item = self.model().item(i,0)
                    return item.checkState() == QtCore.Qt.Checked
   
   
   
            class MyApp(QtWidgets.QMainWindow, Ui_First_MainWindow):
                def __init__(self):
                    QtWidgets.QMainWindow.__init__(self)
                    Ui_First_MainWindow.__init__(self)
                    self.setupUi(self)
                    self.First_pB.clicked.connect(self.Second_EntryForm)
   
   
                def Second_EntryForm(self):
                    dialog = QtWidgets.QDialog()
                    dialog.ui = Ui_Second_Dialog()
                    dialog.ui.setupUi(dialog)
   
   
                    ComboBox = CheckableComboBox(dialog)
                    for i in range(6):
                        ComboBox.addItem("Combobox Item " + str(i))
   
   
                    dialog.exec_()
                    dialog.show()
   
   
   
            if __name__ == "__main__":
                app = QtWidgets.QApplication(sys.argv)
                app.setStyle(QtWidgets.QStyleFactory.create('Fusion'))  # won't work on windows style.
                window = MyApp()
                window.show()
                sys.exit(app.exec_())