I would like to know if it's possible to retrieve in my code different NSString declared in my Xcode scheme or project setting ... Would be useful if a client have to request some secret URL. He just have to modify the schemes or settings.
What you want is environment variables. a preprocessor definition. You can set them in the preprocessor macros of the Xcode build settings.
Take a look here as well: Adding a Preprocessor Definition via a Xcode Scheme