Multiple variables Event (UnityEvent<T0,T1>,UnityEvent<T0,T1,T2>,UnityEvent<T0,T1,T2,T3)

Hi, I am trying to make a custom event so I can send more than 1 variable to a function. I know unity has compatibility with UnityEnvent to up to 4 variables but I can't figure how it works.
I don't understand how it works. when I assign the functions all the fields are empty.
There is a doc on unity but I can't even make the example one to work
https://docs.unity3d.com/ScriptReference/Events.UnityEvent_2.html

I have created some custom shelves for unity to work with the base event class but I don't understand the custom one.
Can someone post an example code that works so I can see how it's done?
like, name class and script inside. (any call like print(x+y) with x and y input in the event function )