That is the code to register the custom field, you should not be adding that to the
card.php file. You should add that coide using the Code Snippets plugin as described in the developer documentation link from my previous reply.
The code that should be placed in the custom template override file is in the section right before the “Export the Field” section.
As for where you put the override file itself… well, that depends entirely on your setup. If you have a parent and child theme, then yes, that folder would be a correct folder. However if you do not have a child theme that that would be the wrong folder because there is no child theme. When I do custom work, I place the files in this folder: