class ViewController: UIViewController, WKUIDelegate {

var webView: WKWebView!

override func loadView() {
    let webConfiguration = WKWebViewConfiguration()
    webView = WKWebView(frame: .zero, configuration: webConfiguration)
    webView.uiDelegate = self
    view = webView
}

// #1 variant
func webView(webView: WKWebView, willSendRequestForAuthenticationChallenge challenge:
    URLAuthenticationChallenge, completionHandler: (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {
    let user = "user"
    let password = "pass"
    let credential = URLCredential(user: user, password: password, persistence: URLCredential.Persistence.forSession)
    challenge.sender?.use(credential, for: challenge)
}

// #2 variant
func webView(webView: WKWebView, didReceiveAuthenticationChallenge challenge: URLAuthenticationChallenge, completionHandler: (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {

        let user = "user"
        let password = "pass"
        let credential = URLCredential(user: user, password: password, persistence: URLCredential.Persistence.forSession)
        challenge.sender?.use(credential, for: challenge)

}

override func viewDidLoad() {
    super.viewDidLoad()

    let myURL = URL(string: "https://myurl.com")
    let myRequest = URLRequest(url: myURL!)
    webView.load(myRequest)
}