Mock external dependency that returns a Future of list

I have a class which have external dependency that returns future of lists. How to mock the external dependency?

 public void meth() {
     //some stuff
     Future<List<String>> f1 = obj.methNew("anyString")
     //some stuff
 }

 when(obj.methNew(anyString()).thenReturn("how to intialise some data here, like list of names")