Avalonia/samples/IntegrationTestApp/App.axaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="IntegrationTestApp.App">
    <Application.Styles>
        <FluentTheme />
    </Application.Styles>
  <TrayIcon.Icons>
    <TrayIcons>
      <TrayIcon Icon="/Assets/icon.ico">
      </TrayIcon>
    </TrayIcons>
  </TrayIcon.Icons>
</Application>