Q. Why is there no mention of Chanuka when reciting the Beracha Acharona for Mezonot or wine? If someone wants to say it anyway is it correct?

A. Mishna Berura (682: 2) rules that on Chanuka or Purim when reciting the Brocho Acharono for Mezonos or wine we don’t mention the special names of these particular days.

Levush, (208) explains that ‘Al Hanissim’ should be recited in a brocho of ‘Hoda’a, or special thanks, that does not include the short Beracha Acharona for Mezonos.

Lechem Chamudos (See Machatzis Hashekel – 208: 18) adds that since these are festivities not mentioned in the Torah, they are not included in this brocho.
However, Piskei Teshuvos (682: 3) mentions that if said, after the fact it is acceptable.

Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a