# authenticate(using:callback:preferredBrowserSession:additionalHeaderFields:) Begins a web authentication session. ``` @MainActor func authenticate(using url: URL, callback: ASWebAuthenticationSession.Callback, preferredBrowserSession: WebAuthenticationSession.BrowserSession? = nil, additionalHeaderFields: [String : String]) async throws -> URL ``` ## Parameters `url` A URL beginning with HTTP or HTTPS that points to the authentication webpage. `callback` An object that describes when the session calls its completion handler. `preferredBrowserSession` The preferred data-sharing behavior of the browser session. For more information, see [`WebAuthenticationSession.BrowserSession`](/documentation/AuthenticationServices/WebAuthenticationSession/BrowserSession). `additionalHeaderFields` A dictionary containing additional header fields to send when loading the initial URL. ## Discussion This initializer throws an error if `additionalHeaderFields` includes a “forbidden request-header” as defined by the WHATWG. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)