How to set AbsoluteLayout.LayoutBounds OnPlatform in XAML file?

How to set AbsoluteLayout.LayoutBounds OnPlatform in XAML file?

I have tried to do like this:

<AbsoluteLayout>
    <Label>
        <AbsoluteLayout.LayoutBounds>
             <OnPlatform x:TypeArguments="Rectangle" iOS="..." Android="..." /> 
            </AbsoluteLayout.LayoutBounds>
    </Label>
</AbsoluteLayout>

but it doesn't work