If you have two separate targets you can place these defines in the project properties its self. To do that you go to your project properties. Click on the target you want to edit, click on the "Build Settings" tab and search for Preprocessor Macros. Define anything you want there and it will be visible for every class in that target.
Another option is to use the same Preprocessor Macros build setting and set a macro for STAGE
. then in your Constant.h you can have something like:
//for stage
#ifdef STAGE
#define SERVICE_URL @"myStageUrl.com"
#else
//for prod
#define SERVICE_URL @"myProdUrl.com"
#endif