Add required "agree to terms" to checkout

Hi,

I’m trying to modify the Authorize.net pro form. On the checkout page, I need an “Agree to terms” checkbox that’s required before submitting the checkout form.

I’ve looked through all of the documentation, and everyone recommends adding a new field in General Options > Registration/Profile Fields, but there is no “add a new field” option on either the General Options nor the Auth.net Pro-Forms page.

Please note that I do not wish to add this required checkbox to the signup form. I’m trying to add it to the checkout form, at the bottom of the credit card and billing info. I realize that I’ll have to modify the Authorize.net template (authnet-checkout-form.php), but I’m not sure what to put in there, and where.

Thanks for your help.