Zoho CRM Web2Lead form, Can we add custom script in javascript[client side]?

Zoho CRM Web2Lead form, Can we add custom script in javascript[client side]?

Actually Some of my privacy, I put some fields in hidden. So I have to fill that field at the time of submitting entire page.
Else I need to update the field in onclick of the submit button.
I have my own java script function. But if I put into the form, the entire form's behavior has changed. Like the redirection is working properly. I am not able to get mail too.

Please tell me, How to add a custom script into the Zoho CRM Web2Lead's form.

Thanks in Advance,