Does a high score in Math subject GRE helps in applying to a Finance PhD program?
While a strong GRE score certainly won't hurt your chances of admission, I think that in general students place too much emphasis on test scores relative to their actual importance in the admissions process.

The only case where this will make a substantial difference is in an "edge" case—where you're on the borderline of being admitted or not and the small "bump" could cause a significant change in the outcome. (If you were a clear admit, the GRE score won't make a difference, and if your candidacy is otherwise weak, a strong GRE score won't rescue it.)

As for how to highlight it, I would just put that information in a "miscellaneous comments" section of the form, if it's available.