As a Square user the checkbox was found under banking settings. It was fairly easy to do on the portable terminal. However it's not just that simple because, depending on your square setup/workflow, you have to change your checkout settings. I had to chamge mine to ask the user what they want to pay with (bitcoin/credit/cash) instead of going straight to credit.

I also have a suspicion that not all terminals have access to the banking setting, again depending on the workflow. You don't want every terminal to have admin rights.