The following sample code for the Navigation Back action is an InlineAction that instructs the FormsManager to close the current screen and navigate to the previous screen:
Sybase.UnwiredPlatform.Windows.Action.Action action0 = Sybase.UnwiredPlatform.Windows.Action.ActionFactory.CreateInlineAction( delegate(Object[] args,out string message ) { FormBase.FormsManager.CloseForm(); message = ""; return false; } ); action0.Execute();