I also tried to put new UXWindow element into UXDesktop, but there was no change. Here is XAML:

<Intersoft:UXPage x:Class="ClientUIApplication1.MainPage" 
	xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	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"
	xmlns:Intersoft="http://intersoft.clientui.com/schemas"
    xmlns:my="clr-namespace:ClientUIApplication1"
	Title="MainPage Page"
	d:DesignWidth="640" d:DesignHeight="480">

    <Intersoft:UXDesktop>
        <Intersoft:UXWindow Header="UXWindow Title" Height="157" Width="336" IsClientVisible="True" CanMaximize="True">
            <Grid />
        </Intersoft:UXWindow>
    </Intersoft:UXDesktop>
</Intersoft:UXPage>

You could use the Intersoft ClientUI Desktop Template to see how the maximize works. In your case, you will need to implement the Maximize executed event handler to handle the maximize process yourself.

Here is a snippet on how to implement Maximize executed on the Settings.xaml which is a UXDialogBox:

public Settings()
{
    InitializeComponent();

    // Create a CommandBinding and attaching an Executed and CanExecute handler
    CommandBinding maximizeCommandBinding = new CommandBinding(
        WindowCommands.Maximize,
        MaximizeExecuted);

    // Create the CommandBindingCollection to hold the command binding
    CommandBindingCollection bindingCollection = new CommandBindingCollection();
    bindingCollection.Add(maximizeCommandBinding);

    // Set the binding collection to the layout root
    CommandManager.SetCommandBindings(this, bindingCollection);

}

private void MaximizeExecuted(object sender, ExecutedRoutedEventArgs e)
{
    this.Height = ((UXPage)this.GetRootVisual()).ActualHeight;
    this.Width = ((UXPage)this.GetRootVisual()).ActualWidth;
    this.Left = 0;
    this.Top = 0;

    this.WindowState = Intersoft.Client.UI.Aqua.UXDesktop.WindowState.Maximized;
}