gistfile1.m

    RKObjectMapping *requestMapping = [RKObjectMapping requestMapping];
        [requestMapping addAttributeMappingsFromDictionary:@{
         @"email": @"email",
         @"username": @"username",
         @"password": @"password",
         }];
        endPoint = kUTAPIEndPointAuthenticate;

    RKRequestDescriptor *requestDescriptor = [RKRequestDescriptor requestDescriptorWithMapping:requestMapping objectClass:[User class] rootKeyPath:@"User"];    

// Serialize
    NSError* error;
    NSDictionary *parameters = [RKObjectParameterization parametersWithObject:user requestDescriptor:requestDescriptor error:&error];
    NSData *serializedUser = [RKMIMETypeSerialization dataFromObject:parameters MIMEType:RKMIMETypeFormURLEncoded error:&error];
    
    // Request
    NSMutableURLRequest *request = [[RKObjectManager sharedManager] requestWithObject:user method:RKRequestMethodPOST path:endPoint parameters:nil];
    [request setValue:RKMIMETypeFormURLEncoded forHTTPHeaderField:@"Content-Type"];
    [request setHTTPBody:serializedUser];
    
    // Succes Failure
    RKManagedObjectRequestOperation *operation = [[RKObjectManager sharedManager] managedObjectRequestOperationWithRequest:request
                                                                                                      managedObjectContext:user.managedObjectContext
    success:^ (RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {
       [self requestDidComplete:operation.HTTPRequestOperation.request];
       success(mappingResult, nil);
    } failure:^ (RKObjectRequestOperation *operation, NSError *error) {
       [self requestDidFail:operation.HTTPRequestOperation.request error:error];
       success(nil, error);
    }];
    
    // Go!
    [[RKObjectManager sharedManager] enqueueObjectRequestOperation:operation];

Discovered two things for this to work correctly.

1. Must obtain a permanent ID

   // Permanent ID
   __block BOOL _blockSuccess;
   __block NSError *_blockError;

   [[user managedObjectContext] performBlockAndWait:^{
   _blockSuccess = [[user managedObjectContext] obtainPermanentIDsForObjects:@[user] error:&_blockError];
   }];

2. Must set the target object of the operation
   operation.targetObject = user;