Issue with Ticket Creation using iOS SDK & setting JSON properties

We are using the iOS SDK to allow our users to create tickets in zendesk with the click of a button. We do not want them to fill out a webform or use chat - rather, click a button and an automatic ticket with the correct properties should be created in Zendesk Support. 

We're all set up, but we seem to be unable to set the JSON properties brand_id and group_id at ticket creation. We only appear to be able to set subject, tags, description, attachments, custom fields, ticket Form ID

consistent with this article we tried setting using the below code but it is failing: 

var client = ZAFClient.init();
client.set('ticket.group_id', 'groupid');

is it simply not possible to set these types of properties using ZDKRequestProvider and ZDKCreateRequest ? are we missing something? why would it be possible to set custom fields and ticket form but not systemic properties?