Have you tried using the default implementation Browser Class AutoSelectBrowser?
AutoSelectBrowser is the default if you do not override the Browser option. It will create the appropriate UI based on the iOS Version
// For iOS 12+ use ASWebAuthenticationSession
if (UIDevice.CurrentDevice.CheckSystemVersion(12, 0))
return ASWebAuthenticationSessionBrowser.Start(options);
// For iOS 11 use SFAuthenticationSession
if (UIDevice.CurrentDevice.CheckSystemVersion(11, 0))
return SFAuthenticationSessionBrowser.Start(options);
// For iOS 10 and earlier use SFSafariViewController
return SFSafariViewControllerBrowser.Start(options);
Only the SFSafariViewController is a popup of safari. The others are embedded WebViews. They also display an alert to the user. This alert gives the user an indication that they are being redirected.
Hope this helps.