WinUI3 is the technology used by MAUI on Windows, and you can refer to Windowing overview for WinUI and Windows App SDK to understand the link between a WinUI3 Window to a Win32 HWND.

Then, you can use Platform lifecycle events to get it, something like this:

using Microsoft.Maui.LifecycleEvents;

namespace MauiApp1
{
    public static class MauiProgram
    {
        public static MauiApp CreateMauiApp()
        {
            var builder = MauiApp.CreateBuilder();
            builder
                .UseMauiApp<App>()
                .ConfigureLifecycleEvents(events =>
                {
#if WINDOWS
                    events.AddWindows(bld =>
                    {
                        // window here is of Microsoft.UI.Xaml.Window type (WinUI3)
                        bld.OnWindowCreated(window =>
                        {
                            var hwnd = Microsoft.UI.Win32Interop.GetWindowFromWindowId(window.AppWindow.Id);
                        });
                    });
#endif
                });

            return builder.Build();
        }
    }
}