Add phone and custom fields to custom ticket form with the Ticketing API

From what I understand we are limited to https://developer.zendesk.com/documentation/ticketing/managing-tickets/creating-and-managing-requests/#creating-anonymous-requests

I added my custom form inputs into the comment body and the agent needs to manually add it to the custom field...

ACO Is your query meant to address the pre-population of a customised user field such as mobile number through the API? If yes I have the same query and really keen to be directed to the right documentation to achieve this.