Adds one option to an existing dropdown, listbox, radio, or checkbox custom field. If the field already has an option with the same value, no new option is created and the existing option is returned with status 200 instead of 201. label defaults to value. The field id in the payload is not checked against existing fields, so a wrong id silently creates an option that never appears on any field; confirm it with GET /fields/{id} first. The 201 response echoes the property types as sent; subsequent reads return every property as a string. To create several options in one call use POST /fieldOption/bulk.