MarkFullscreenWindow, 2 implementations.

ITaskbarList2_MarkFullscreenWindow.c

BOOL MarkWindowFullscreen(HWND hWnd, BOOL fFullscreen)
{
    ITaskbarList2 *pTaskbarList2 = NULL;
    CoInitializeEx(NULL, COINIT_APARTMENTTHREADED | COINIT_DISABLE_OLE1DDE);
    CoCreateInstance(&CLSID_TaskbarList, NULL, CLSCTX_INPROC_SERVER, &IID_ITaskbarList2, (LPVOID *)&pTaskbarList2);

    pTaskbarList2->lpVtbl->HrInit(pTaskbarList2);
    HRESULT hr = pTaskbarList2->lpVtbl->MarkFullscreenWindow(pTaskbarList2, GetConsoleWindow(), TRUE);
    pTaskbarList2->lpVtbl->Release(pTaskbarList2);

    return SUCCEEDED(hr);
}

MarkFullscreenWindow.c

BOOL MarkFullscreenWindow(HWND hWnd, BOOL fFullscreen)
{
    if (!GetShellWindow() || !IsWindow(hWnd))
        return FALSE;
   
    UINT uMsg = RegisterWindowMessageW(L"SHELLHOOK");
    WCHAR szClassName[256] = {};
    HWND hWndChildAfter = NULL;
   
    while ((hWndChildAfter = FindWindowExW(NULL, hWndChildAfter, NULL, NULL)))
    {
        RealGetWindowClassW(hWndChildAfter, szClassName, 256);
        if (CompareStringOrdinal(L"WorkerW", -1, szClassName, -1, FALSE) == CSTR_EQUAL)
            PostMessageW(hWndChildAfter, uMsg, fFullscreen ? 0x35 : 0x36, (LPARAM)hWnd);
    }

    return TRUE;
}