What is the supplication a woman recites when performing ghusl after menstruation, and what is the ruling on one who forgets to recite it: does she have to perform ghusl again?

There is no specific dhikr or supplication for ghusl (ritual bath) after menstruation or postpartum bleeding, except for the recommendation to say "Bismillah" (In the name of Allah) at the beginning of the ghusl. The obligations for ghusl are: the emission of semen, the cessation of menstrual or postpartum bleeding, and the meeting of the two circumcised parts (intercourse).