Ability to edit 'onSubmit' string of the form button

It would be great to have ability to add something to string "onSubmit" in button code in the contact form (here http://c2n.me/j064dz.png ) ? E.g. several statistics service (Yandex.Metrica, for instance) requests this code is to add in this string something like

onsubmit="yaCounter(counter_number).reachGoal('target_tag'); return true;"

Apparently now I can do it manually but in this case I will need to change code every time I update page.

Thanks.