# additionalHeaderFields Additional headers to send when loading the initial URL. ``` var additionalHeaderFields: [String : String]? { get } ``` ## Discussion Apply these headers only to the initial page, and don’t overwrite any headers that the browser usually sends. This example shows the call to the [`begin(_:)`](/documentation/AuthenticationServices/ASWebAuthenticationSessionWebBrowserSessionHandling/begin(_:)) delegate method when a web browser receives a request from an app. This implementation iterates over the additional header fields from the app’s request and adds them to its own request. ```swift func begin(_ request: ASWebAuthenticationSessionRequest!) { self.request = request let urlRequest = URLRequest(url: request.url) // Use the new fields on the URL request. for (header, value) in request.additionalHeaderFields { urlRequest.addValue(value, forHTTPHeaderField: header) } // Load the URL request. } ``` > Important: Your browser app needs to add `AdditionalHeaderFieldsAreSupported` with the value `YES` to the `ASWebAuthenticationSessionWebBrowserSupportCapabilities` dictionary in your app’s information property list to use this API. If the system doesn’t find this key in the default browser app, it sends the request to Safari instead. --- 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)