How to use Device.OnPlatform for to provide just iOS and default behavior

So I have been looking at This link

and still can't see how I am Supposed to use Device.OnPlatform. I would like essentially a different behvaior for iOS and all other platforms to perform the default.

So far I have:

Device.OnPlatform(
   iOS: () => {//Do my thang},
   default: () => {//But I don't Work});

What am I doing wrong?