How to link a slot in the code with a QWidget in mainwindow

Question

I am a beginner with signal & slots in QtCreator.

I would like to know how to connect a signal declared in mainwindow code in a Qwidget LCDNumber include in the mainwindow.

Because even if my ManWindow signal is diclared correctly (I guess)

I can't configure the connection between MainWindow signal and LDCNumber Display(int) SLOT it in design mode :

Would you know how to code the connection of MainWindow SIGNAL declared in MinWindow's Class and LCDNumber include in Mainwindow's SLOT, please?

Here are all the propeties of QLCDNumber class :

Thanks a lot!!