When to pass a parameter as a constructor parameter or as a method parameter

Question

I currently have this class that uses a Service class in a class method. Currently I am creating an instance of the service class in my method as shown below. The question I am wondering is that when it comes to testing, I am using spock and it seems to be difficult to test when a new instance of a class is being created in the method rather than being passed in as a constructor parameter for dependency injection. I am wondering would passing in an instance of Service class into Handler as a constructor parameter be the correct way of doing this? Thanks

public class Handler{
   private Service service;

    public Handler(){}

public void someMethod(ObjectNeededForService object){
   service = new Service(object);
}
}