How to use callback response parameter in another class using that mathod

Im trying to create a generic pooling system, from a different class Im trying to get an Item, if it dosent exist I need to selectively get it from server? How can I selectively(GetX,GetY) can pass the response callback Item

public class ObjectPool{

    private List<Item> pool = new List<Item>();

    public Item GetItem(Action<object,string> GetNewItem){
        if(!itemExist){
            GetNewItem((newItem)=>{
                pool.add(newItem);
            },(error)=>{
                //error handling
            });
         }
        return pool.First();
    }
}


public class SomeClass() {

    private ObjectPool myPool = new ObjectPool();

    public void GetX(){
        myPool.GetItem(NetworkService.GetX)
    }
    public void GetY(){
        myPool.GetItem(NetworkService.GetY)
    }
}


public static class NetworkService{
    GetX(Action<object>onSuccess,ActionData<string> OnFail){
          //do something
    }

    GetY(Action<Object>onSuccess,ActionData<string> OnFail){
        //do something else
    }
}

How can I receive newItem inside ObjectPool class