how to pass function as parameter in unity code example

Example: adding a function with a parameter to a button Unity

void Start ()
      {
          ButtonPre.onClick.AddListener(delegate{SwitchButtonHandler(0);});
          ButtonNext.onClick.AddListener(delegate{SwitchButtonHandler(1);});
      }
  
      void SwitchButtonHandler(int idx_)
      {
          //Here i want to know which button was Clicked.
          //or how to pass a param through addListener
      }