This API is not yet available publicly. I will share this request with the Freshdesk product team so they can prioritize it accordingly. For now, I would suggest the alternate manual procedure you outline above.
8 days ago
We're working on building the API to create a ticket field in addition to the already available API to list all ticket fields in an account. So for now, you'll have to create the field via the UI and then access it's ID etc via the List Ticket Fields API and connect it in the app.
I'll update you here once its in beta / goes live.
Freshdesk Product Management
about 8 hours ago
besides creating a new custom field, will it also be possible to update such a field?
We´d like to keep those fields up to date with information from AD and other sources.