WinUI API Reference – Microsoft Docs

<Window
    x:Class="MyApp.MainWindow">
    <Grid>
        <StackPanel HorizontalAlignment="Center"
                    VerticalAlignment="Center"
                    Spacing="12">
            <Button Content="Click me"
                    Click="OnClick"/>
            <TextBox x:Name="Output"
                     Width="200"
                     IsReadOnly="True"/>
        </StackPanel>
    </Grid>
</Window>

private void OnClick(object sender, RoutedEventArgs e)
{
    Output.Text = "Button clicked!";
}