func webView(webView: WKWebView, didReceiveAuthenticationChallenge
           challenge: NSURLAuthenticationChallenge,
   completionHandler: (NSURLSessionAuthChallengeDisposition, NSURLCredential?) -> Void) {

    let creds = NSURLCredential(user:"username", password:"password", persistence: NSURLCredentialPersistence.ForSession)
    completionHandler(NSURLSessionAuthChallengeDisposition.UseCredential, creds)
}

Authorization: Basic QWxhZGRpbjpPcGVuU2VzYW1l

extension MyController: WKNavigationDelegate {
    func webView(_ webView: WKWebView, didReceive challenge: URLAuthenticationChallenge, completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {
        guard let u = self.webuser, let p = self.webp else {
            completionHandler(.cancelAuthenticationChallenge, nil)
            return
        }

        let creds = URLCredential.init(user: u, password: p, persistence: .forSession)
        completionHandler(.useCredential, creds)
    }
}