Linker error when try to call ObjC method from C

Actually, I'm a Unity developer and I know nothing about XCode and Objective C. But I have to use ObjC library for some integration and call this function from Unity. So I import that library to created XCode project and write some wrapper code between Unity and XCode. When I try to call empty function, it works for me. But when i try to write ObjC method in C file it causes linker error and I have no idea what's wrong with it. So, here's the deal:

In Unity MonoBehaviour.cs:

[DllImport("__Internal")]
public static extern void startWithAPIKey(string apiKey);
...
startWithAPIKey(apiKey);

In XCode Wrapper.h:

@interface IOSYandexMetricaWrapper : NSObject
@end

In XCode Wrapper.m:

#import "YandexMobileMetrica.h" //library header
#import "IOSYandexMetricaWrapper.h"

void startWithAPIKey(char *apiKey) {
    //method that i want to call
    [YMMYandexMetrica startWithAPIKey:(NSString *)apiKey];
}

So it compiles, but linker says something like this: Undefined symbols for architecture armv7: "_OBJC_CLASS_$_YMMYandexMetrica", referenced from: objc-class-ref in IOSYandexMetricaWrapper.o

Someone can help me with my problem?