Unable to get cookies for flutter iOS webview
The plugin that you're using (flutter_inappbrowser) seems to be outdated and has been discontinued. I suggest using webview_flutter as it has support for this feature and has just been recently out of developer preview.
To fetch cookies using webview_flutter, you can call WebViewController.evaluateJavascript('document.cookie');. The method evaluateJavascript is used to evaluate the JavaScript expression 'document.cookie' of the page and returns a String value. You can check the sample in this page to try it out.