Zoho desk extension update custom field

Hi all,

I'm trying to update a custom field in my zohodesk extension. But the documentation is fairly unclear.

The custom field is named "testveld". The api name is cf_testveld

I'm trying to update it like this:

ZOHODESK.set('ticketForm.cf_testveld', { value: "demotext"});

I get a succes message back. 
When I try to set it like this:
ZOHODESK.set('ticket.cf_testveld', { value: "demotext"});

I get: {errMsg: 'No module/data found to perform set'}

But in both cases the field is not updated. Can someone help me on how I should set a custom incident field?>