add a dropbox to the payment method credit card

-- HikaShop version -- : 2.5.0
-- Joomla version -- : 3.4.0

Hello I've bought your business version of Hikashop and I'd like to know how can I add a dropbox with 3 options on payment method credit card, bellow the ccv input field. I need a select box there with 3 options for installment, the options will be 1x, 2x or 3x. and that info needs to be sent through email to the admin.
I've tried to change the view "ccinfo" code, but nothing changes in the frontend, can you help me with this?

Thanks in advance.
Best regards!

I've managed to use a dropbox that was already there the card type, I just have to change its text now and reposition it.
can you tell me which file do I have to change to do this? it's for the credit card payment type.

Thanks in advance!