How to override onsubmit for custom validation

[ltwinnerr]

I want to override the onsubmit attribute of the form on the newthread page as I have added a few custom fields to it that I need to validate. However the onsubmit value will not change. Here's what I am doing -

var form = document.getElementsByTagName("form")[1];
form.onsubmit = 'override_submit()';

And nothing changes. However if I try to change the action attribute of the form it works no problem.

form.action= 'xyz';

So any ideas on what I am doing wrong?