How to store object or string in Objective-C and use it

I'm making a react native app and using a native module to get a "string" inside objective-c ios project.

RCT_EXPORT_MODULE();RCT_EXPORT_METHOD(getEmail:(NSString *)email){  RCTLogInfo(@"Getting %@", email);}

I don't know anything about the Objective-C part and I don't know how to store my "email" into something I can use and retrieve inside the AppDelegate.m

Example : [customFieldsDictionary setObject:email forKey:@"EMail"];

If someone can help or guide me through this, it would be awesome !

Thank you