Using BoxView you can draw a rectangle.Below is a Example of Rectangle whose background color is red.
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:d="http://xamarin.com/schemas/2014/forms/design"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
x:Class="ImageSlider.Views.BoxView">
<Grid>
<BoxView BackgroundColor="Red" HeightRequest="120" VerticalOptions="Start" HorizontalOptions="Fill" Margin="10">
</BoxView>
</Grid>
</ContentPage>