Credit Card Total Amount field

I'm trying to configure a payment form on my website where customers can enter their own amount to pay their bill pay via Credit Card. I can't seem to add a Custom field for the amount which will then be added automatically to the "Total $ Amount Box" using the authorize.net option. Can you help with this? The website is here: www.homesteadfuel.com/services/pay-online.html
Thank you. love the product so far!

I think I figured it out. Jquery needed to be set to ON in the General Configuration.

Yes Brian. The extension needs jQuery library loaded, so unless it is loaded by your template already, you will need to set to ON in Configuration

Tuan