How to add a custom field

Hi,

Can I add a custom field to the payment option? I need a field called “Reference Number” so I can know who made a payment. The field doesn’t have to be in the popup window that comes after you press “Pay Now” or “Buy Now” button, it can be visible on the initial page where the button is, just above the button where the amount is, like here: http://prntscr.com/kmqy03.

As long as I can see in my confirmation email or my dashboard or anywhere what the reference number is for the payment it will be fine.

Thanks,
Anja