How to add a new product field on a current payment form

To add a new product, first click on the payment section, then click on the wizard icon: 

Once you're there you will get a window to configure your payment integration, it will variate depending on the payment gateway you've integrated, in my example I used the Purchase Order method, there you will need to click on the continue button:

You will get a window, click on next as shown below:

You will get the products section, there you can add more products, once you've finished only click on the next button and save changes. 

Hope this helps.