viewController.swift

import UIKit

extension Segue {
    
  static var signup: Segue = Segue(identifier: "signup")
    
}

class OBEmailViewController: UIViewController {
  
  // Some code
  
  // Actions
  @IBAction func next(_ sender: Any) {
     perform(segue: .signup)
  }
  
  // Navigation
  override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
     // That way is straight forward without grammar mistakes
     if segue.equal(.signup), 
        let vc = segue.destination as? OBSignupViewController {
           vc.email = emailTF.text
     }
  }
  
  // Some code
  
}