oap_labs/AvaloniaApplication1/MainWindow.axaml

<Window 
    xmlns="https://github.com/avaloniaui"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d" 
    d:DesignWidth="800" 
    d:DesignHeight="450"
    x:Class="AvaloniaApplication1.MainWindow"
    Title="AvaloniaApplication1"
    xmlns:local="clr-namespace:AvaloniaApplication1"
>
    <Button 
        x:Name="phoneButton"  
        Width="250" 
        Height="40"
    >
        <local:Phone
            Name="Lumia 950" 
            Price="700" />
    </Button>
</Window>