hey quentin!!
i released some of the tutorial online now:
https://laytheme.com/tutorials/ai-mode.html#enable-ai-mode
this point describes the way of doing it:
https://laytheme.com/tutorials/ai-mode.html#custom-fields
Having trouble myself sorry @ray
Also because Ninjaforms is a third party plugin i wont be able to devote much time to sorry - ( although i know many users have ninja forms... )
Best
Richard