A demo extension with a short step by step manual is available on GitHub https://github.com/derhansen/sf_event_mgt_extend_demo

Extending FlexForm setting “settings.orderField”

If you need to add a new option to the FlexForm setting “settings.orderField”, please read this blogpost How to extend existing FlexForm select options of a TYPO3 plugin using Page TSconfig which shows this task as an example.