Question:

Do u have to say over ahmonei esrei by mariv if u forgot al hanisim

Answer:

Whenever al hanisim is forgotten, whether it is by davening or by bentching if we already said the words Boruch ata H-shem at the end of the bracha, then we don’t go back. If we remembered before that we do go back.

Purim Sameach

Rabbonim of the Beis Hora'a