AppDelegate.m

#import "AppDelegate.h"

#import <React/RCTBundleURLProvider.h>
#import <React/RCTRootView.h>
#import <ReactNativeNavigation/ReactNativeNavigation.h>

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
  NSURL *jsCodeLocation = [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index" fallbackResource:nil];
  self.moduleRegistryAdapter = [[UMModuleRegistryAdapter alloc] initWithModuleRegistryProvider:[[UMModuleRegistryProvider alloc] init]];
  [ReactNativeNavigation bootstrap:jsCodeLocation launchOptions:launchOptions bridgeManagerDelegate:self];
  return YES;
}

- (NSArray<id<RCTBridgeModule>> *)extraModulesForBridge:(RCTBridge *)bridge
{
  NSArray<id<RCTBridgeModule>> *extraModules = [_moduleRegistryAdapter extraModulesForBridge:bridge];
  // You can inject any extra modules that you would like here, more information at:
  // https://facebook.github.io/react-native/docs/native-modules-ios.html#dependency-injection
  return extraModules;
}

- (NSURL *)sourceURLForBridge:(RCTBridge *)bridge {
#ifdef DEBUG
  return [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index" fallbackResource:nil];
#else
  return [[NSBundle mainBundle] URLForResource:@"main" withExtension:@"jsbundle"];
#endif
}

@end

More complete code here: https://gist.github.com/brentvatne/1ece8c32a3c5c9d0ac3a470460c65603