Hi there Tom. At this point the custom fields are limited to text; while different methods of displaying that text are available (dropdown, checkbox, text field, etc) there isn’t an option for a WYSIWYG editor at this time. Though, that may come down the road. If you’re interested in seeing more on the custom fields and how they’re used, check out this tutorial: https://theeventscalendar.com/pro-adding-custom-events-attributes/.
Hope that helps. If you have any other questions, please let me know. Thanks again for your interest so far.