Show custom field in attendee list

Currently I am using the default code [rtec-attendee-list event=1000] to show the already registered attendees of a planned event. The default will only show the names. From older posts here, I found that I should use the action described here
https://roundupwp.com/codex/action-rtec_the_attendee_list/

I have been adding the mofified code at the end of the file form-functions.php, hoping this would display the custom field I added to the registration form, but this resulted in an error when displaying the page.

Can someone please guide me where to add the code? Thanks a lot in advance!