Custom signal in qt designer

I am using qt-designer intside qt-creator to create the main window of my application. I need a button that emits a signal and a label that changes with this signal. My idea was to connect button's signal to a MainWindow and, from there, send another signal to the label. This way my MainWindow also knows that the button has been clicked.

When I was doing this, I have seen that I can add a new custom slot to my MainWindow using the designer and I have done this. But I can't find where I have to write the code for this slot. Can anybody help me?

Thanks in advance!

PS: When I execute the program, it generates the following output:

Object::connect: No such slot MainWindow::Slot1()