IBOutlet link to embedded container view controller

Another option for some cases is to capture the embedded controller using -prepareForSegue:sender:.

For example, if I have a UINavigationController embedded within a CustomContainerViewController, I can name the embed segue embedContentStack in the storyboard and capture it in CustomContainerViewController via

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    if ([segue.identifier isEqualToString:@"embedContentStack"]) {
        // can't assign the view controller from an embed segue via the storyboard, so capture here
        _contentStack = (UINavigationController *)segue.destinationViewController;
    }
}