She is asking about a specific du’aa’ for when she finishes her period

If the bleeding stops and she become pure, and she does
ghusl after menstruation with the intention of purifying herself, then
she is considered to be pure (taahirah). There is no specific du’aa’
for that, apart from the well known du’aa’s that have been narrated
for doing wudoo’. And Allaah knows best.