gistfile1.m

// make the post using the objectManager if you want to map the response to a model
RKObjectManager* objectManager = [RKObjectManager sharedManager];  
[objectManager loadObjectsAtResourcePath:@"/api/" delegate:self block:^(RKObjectLoader* loader) {
    loader.serializationMIMEType = RKMIMETypeJSON; // We want to send this request as JSON
    loader.objectMapping = [objectManager.mappingProvider objectMappingForClass:[Plan class]];
    loader.resourcePath = @"/api/";
    loader.method = RKRequestMethodPOST;
    loader.params = [RKRequestSerialization serializationWithData:[jsonString dataUsingEncoding:NSUTF8StringEncoding] MIMEType:RKMIMETypeJSON];
}];