Edit HTML of forms? Add autocomplete="off"?

Edit HTML of forms? Add autocomplete="off"?

Recently I've noticed that something — one customer suggested it was Chrome — is auto-filling one of my form fields with the customer's address. The field is a textarea that has nothing to do with the address — it's just a comment field. I'd like to be able to add autocomplete="off" to the textarea or input tags on the form. Is there any way to do that? If not, why not and when?