Avalonia/samples/IntegrationTestApp/Pages/EmbeddingPage.axaml

<UserControl 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"
             xmlns:embedding="using:IntegrationTestApp.Embedding"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="IntegrationTestApp.EmbeddingPage">
  <StackPanel>
    <embedding:NativeTextBox Name="NativeTextBox" Height="23"/>
    <StackPanel Orientation="Horizontal">
      <CheckBox Name="EmbeddingPopupOpenCheckBox">Open Popup</CheckBox>
      <Popup IsOpen="{Binding #EmbeddingPopupOpenCheckBox.IsChecked}"
             PlacementTarget="EmbeddingPopupOpenCheckBox"
             Placement="Right">
        <embedding:NativeTextBox Name="NativeTextBoxInPopup" Width="200" Height="23"/>
      </Popup>
    </StackPanel>
  </StackPanel>
</UserControl>