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 = {};
    pw.pid = 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