See the question and my original answer on StackOverflow

Only Window implements IWindowNative, so you need to pass the window reference around, or if you're sure there's only one Window in your process, you can use a code like this:

HWND GetProcessFirstWindowHandle(DWORD pid = 0)
    struct ProcessWindow { DWORD pid;  HWND hWnd; } pw = {}; = pid ? pid : GetCurrentProcessId();
    EnumWindows([](auto hWnd, auto lParam)
            DWORD pid;
            GetWindowThreadProcessId(hWnd, &pid);
            if (pid != ((ProcessWindow*)lParam)->pid)
                return TRUE;

            ((ProcessWindow*)lParam)->hWnd = hWnd;
            return FALSE;
        }, (LPARAM)&pw);
    return pw.hWnd;

And for example, call it simply like this:

void MainWindow::myButton_Click(IInspectable const&, RoutedEventArgs const&)
    auto hwnd = GetProcessFirstWindowHandle();

You can also add some check on class name, like what's done in this answer (it's C# but the code is already using interop to access native Windows APIs) : Retrive Window Handle in Class Library WinUI3