AvaloniaProject/AvaloniaApplication4/Views/MessageBox.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="400" d:DesignHeight="250"
		CanResize="False" MaxHeight="250" MaxWidth="400" MinHeight="250" MinWidth="400"
        x:Class="AvaloniaApplication4.Views.MessageBox"
        Title="Сообщение"
		Padding="20"
		Background="White">
	<StackPanel Orientation="Vertical">
		<TextBlock Text="Уведомление" TextAlignment="Center" Margin="0 0 0 30" FontWeight="Bold" FontSize="14" TextWrapping="Wrap"/>
		<TextBlock Name="message" Text="Упс, что-то пошло не так :(" Margin="0 0 0 30" FontSize="12" TextWrapping="Wrap"/>
	</StackPanel>
</Window>