I want use resource in another resource in Resource dictionary

How to use resouce like this.

    <OnPlatform
        x:TypeArguments="x:String"
        x:Key="SourceSansPro_Black">
        <On
            Platform="iOS"
            Value="SourceSansPro-Black" />
        <On
            Platform="Android"
            Value="SourceSansPro-Black.ttf#Source Sans Pro" />
    </OnPlatform>