Relates a custom field to a list so the field applies to contacts on that list. relid 0 relates the field to every list. If the relationship already exists, the existing record is returned with 200 instead of 201, in a reduced shape without links and with cdate formatted as a plain SQL datetime rather than ISO 8601. To remove a relationship use DELETE /fieldRels/{fieldRelId}; to replace a field's whole set of list relationships in one call use POST /fieldRel/sync or POST /fieldRel/syncExcept.