Subtracting from a variable through button clicks

Hi everyone I'm pretty new at unity (and coding for that matter), but I can't seem to figure out how to make the button subtract from the variable. I've tried adding the script to a game object, making it an OnClick event, but that didn't help.

public class Costs : MonoBehaviour { public int myenergylevel = 40; Selectable button;

 void TaskOnClick()
 {
     button = gameObject.GetComponent<Selectable>();
     Debug.Log(button.interactable.ToString());
     myenergylevel = myenergylevel - 6;
 }

}