How to use subscript in Swift

extension UserDefaults {
    enum Key: String {
        case hasBackup
    }

    subscript(key: Key) -> Bool {
        get {
            bool(forKey: key.rawValue)
        }
        set {
            set(newValue, forKey: key.rawValue)
        }
    }
}

UserDefaults.standard.hasBackup] = true