Get value of App.Config from Xaml file

it mean, i have a App.config containing a value as
<appsettings>
<add key="FlowDirection" value="LTR">


->how to get this value ('LTR') from from Xaml file


<setter property="FlowDirection">
Value="{Binding RelativeSource={RelativeSource Self}, Converter={StaticResource
FlowDirectionConverter}}, ConvertParameter = '...config value....'"/&gt;
</setter>
i want to binding value "LTR" to ConvertParameter