How a QMainWindow can open a dialog?

[panoss]

I have the following code which has two classes:
-Ui_dialog_in
-myWindow

What I 'm trying to achieve is:
-when myWindow opens, to open a Ui_dialog_in

What must I change - add?

from PyQt6 import QtCore, QtGui, QtWidgets
from PyQt6.QtWidgets import QMainWindow, QHBoxLayout, QWidget
from PySide6.QtWidgets import QApplication

class Ui_dialog_in(object):
    def setupUi(self, dialog_in):
        dialog_in.setObjectName("dialog_in")
        dialog_in.resize(400, 146)
        self.verticalLayout_3 = QtWidgets.QVBoxLayout(dialog_in)
        self.verticalLayout_3.setObjectName("verticalLayout_3")
        self.verticalLayout_2 = QtWidgets.QVBoxLayout()
        self.verticalLayout_2.setObjectName("verticalLayout_2")
        self.verticalLayout = QtWidgets.QVBoxLayout()
        self.verticalLayout.setObjectName("verticalLayout")
        self.horizontalLayout_2 = QtWidgets.QHBoxLayout()
        self.horizontalLayout_2.setObjectName("horizontalLayout_2")
        self.titleLabel = QtWidgets.QLabel(dialog_in)
        self.titleLabel.setObjectName("titleLabel")
        self.horizontalLayout_2.addWidget(self.titleLabel)
        self.titleEdit = QtWidgets.QLineEdit(dialog_in)
        self.titleEdit.setObjectName("titleEdit")
        self.horizontalLayout_2.addWidget(self.titleEdit)
        self.horizontalLayout_2.setStretch(0, 1)
        self.horizontalLayout_2.setStretch(1, 4)
        self.verticalLayout.addLayout(self.horizontalLayout_2)
        self.horizontalLayout_3 = QtWidgets.QHBoxLayout()
        self.horizontalLayout_3.setObjectName("horizontalLayout_3")
        self.authorLabel = QtWidgets.QLabel(dialog_in)
        self.authorLabel.setObjectName("authorLabel")
        self.horizontalLayout_3.addWidget(self.authorLabel)
        self.authorNameCombo = QtWidgets.QComboBox(dialog_in)
        self.authorNameCombo.setObjectName("authorNameCombo")
        self.horizontalLayout_3.addWidget(self.authorNameCombo)
        self.horizontalLayout_3.setStretch(0, 1)
        self.horizontalLayout_3.setStretch(1, 4)
        self.verticalLayout.addLayout(self.horizontalLayout_3)
        spacerItem = QtWidgets.QSpacerItem(20, 40, QtWidgets.QSizePolicy.Policy.Minimum, QtWidgets.QSizePolicy.Policy.Expanding)
        self.verticalLayout.addItem(spacerItem)
        self.horizontalLayout = QtWidgets.QHBoxLayout()
        self.horizontalLayout.setObjectName("horizontalLayout")
        spacerItem1 = QtWidgets.QSpacerItem(40, 20, QtWidgets.QSizePolicy.Policy.Expanding, QtWidgets.QSizePolicy.Policy.Minimum)
        self.horizontalLayout.addItem(spacerItem1)
        self.okButton = QtWidgets.QPushButton(dialog_in)
        self.okButton.setObjectName("okButton")
        self.horizontalLayout.addWidget(self.okButton)
        self.pushButton = QtWidgets.QPushButton(dialog_in)
        self.pushButton.setObjectName("pushButton")
        self.horizontalLayout.addWidget(self.pushButton)
        self.verticalLayout.addLayout(self.horizontalLayout)
        self.verticalLayout_2.addLayout(self.verticalLayout)
        self.verticalLayout_3.addLayout(self.verticalLayout_2)

        self.retranslateUi(dialog_in)
        QtCore.QMetaObject.connectSlotsByName(dialog_in)

    def retranslateUi(self, dialog_in):
        _translate = QtCore.QCoreApplication.translate
        dialog_in.setWindowTitle(_translate("dialog_in", "Dialog"))
        self.titleLabel.setText(_translate("dialog_in", "Title"))
        self.authorLabel.setText(_translate("dialog_in", "Author"))
        self.okButton.setText(_translate("dialog_in", "PushButton"))
        self.pushButton.setText(_translate("dialog_in", "PushButton"))



class myWindow(QMainWindow):
    """A window to show the books available"""
    def __init__(self, parent=None):
        """Initializer."""
        super().__init__(parent)
        self.setWindowTitle("My title")
        self.resize(550, 250)
        self.centralWidget = QWidget()
        self.setCentralWidget(self.centralWidget)
        self.layout = QHBoxLayout()
        self.centralWidget.setLayout(self.layout)
        #self.setupUI()
        # open second window, myDialog
        myDialog = Ui_dialog_in()


if __name__ == "__main__":
    app = QApplication([])
    import sys
    window = myWindow()
    window.resize(800, 600)
    window.show()
    sys.exit(app.exec())