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On BB10 is there is a way to prompt a username/password dialog? I've only been able to find examples of simple alerts with optional extra buttons.
No, not without creating your own extension, or making your own with CSS/HTML.
If you'd like more info on creating extensions for BB10 you can check-out this out https://developer.blackberry.com/html5/documentation/creating_extensions_for_bb10_apps.html