Inside of Microsoft Dynamics 365 Business Central, go to the merchant setup window. Turn on the convenience fee parameter. You will have the option to select a flat amount convenience fee or a percent. From there, you will select the GL Account that the fees are going to post to. The offset account will be the account you selected for cash.
All future transactions will have the the convenience fee turned on. You do have the option to turn off the convenience fee for specific transactions of your choosing.