This feature is dependent on the application you are using to process your credit cards.
If you are using Datacap NETePay and would like to add a surcharge, you will need to be moved to Datacap NETePay Hosted (Datacap is only supporting surcharge on their hosted app). The only non-Datacap solution that we have certified with surcharge is PayJunction.
If you would like to learn more, please contact our sales team for additional information. If you are currently doing surcharges or thinking about it, please watch this video to learn more about the rules with which you will need to comply.