//In my View Controller, I have tried this:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "GoToTabBar" {
        let custMainVC = segue.destination as! CustomerMainViewController
        custMainVC.tempName = self.emailTextField.text!

    }
}

//In my tab bar controller whose first View Controller has a name of CustomerMainViewController:

override func viewDidLoad() {
    super.viewDidLoad()
    username.text = self.tempName
}