Reverse a modal segue?

In my app, there's a login screen. On that login screen, there's also a register button. If a user clicks the register button, a modal segue is executed to present a register view.

However, when the user clicks 'cancel', I want to do the same animation but then in reverse. So basically the register view should slide down. I created a custom segue for this, but I don't know how to properly animate this. How can I do this?

ReverseModalSegue.m

#import "ReverseModalSegue.h"

@implementation ReverseModalSegue

- (void) perform {

    UIViewController *src = (UIViewController *) self.sourceViewController;
    [UIView transitionWithView:src.navigationController.view duration:0.5
                       options:UIViewAnimationOptionTransitionFlipFromBottom
                    animations:^{
                        [src.navigationController popToViewController:    [src.navigationController.viewControllers objectAtIndex:0] animated:NO];;
                    }
                    completion:NULL];
}

@end

ReverseModalSegue.h

#import <UIKit/UIKit.h>

@interface ReverseModalSegue : UIStoryboardSegue

@end