What's the "damage" are you trying to change? Is It fall damage, weapon damage, etc...
Between the brackets are called parameters you have to pass digits or string depends on the parameter data type
You either can pass them as numeric values or as variables that holds a value.
As you Registered a hook to the damage event of a player through ham module and provided the callfunction which is (isDamaged), every time the player gets damaged the isDamaged function will be called.
So calling it within the plugin won't make any changes.