I have exercise (picture 1)

It’s study guide for windows forms app but I only have macOS with xamarin cocoa app.

I just don’t know how I can get every change of a textField and do smth (in this case to assign this value to label)

I found a topic with the same problem here, but only pieces of code were provided there. I tried to insert them anywhere, but nothing works.
I couldn’t ask anything either, it takes 50 reputations that I don’t have.

I would appreciate it if you could explain how to do this step by step. I really want to understand every moment