Translated from Ustadz Firanda Andirja's Indonesian commentary (Bekal Islam). Quoted verses, hadith and duas keep their Arabic; where the commentary gave its own Indonesian rendering of them, that rendering is not translated and is marked [translation not shown].
After a voluntary prayer (other than witr and duha)
There is no specific dhikr, so you may make whatever dhikr you wish, without making that dhikr a fixed habit, because there is no authentic evidence indicating a specific dhikr; and if one does not make dhikr, that is no problem either.