Style OnPlatform and DynamicResource

I have defined a style , but I'd like to set the Android and iOS values to a DynamicResource - What's the syntax?

<Style
    TargetType="SearchBar"         ApplyToDerivedTypes="true">
    <Setter Property="BackgroundColor" Value="{OnPlatform Android='White', iOS='#4F8BBF'}"/>
</Style>