This should work if you're hooking the
CreateDialogIndirectParamAorW function.
code:
HWND MyCreateDialogIndirectParamAorW(HINSTANCE hInstance, LPCDLGTEMPLATE lpTemplate, HWND hWndParent, DLGPROC DialogFunc, LPARAM lParamInit)
{
HWND hWndRet = OriginalCreateDialogIndirectParamAorW(hInstance, lpTemplate, hWndParent, DialogFunc, lParamInit);
ShowWindow(hWndRet, SW_HIDE);
return hWndRet;
}