Question: can't create new Custom Fields and append to ticket

[croves]

I need to add Custom Fields to a ticket based on a number of factors. I'm trying to automate this job with the following code:

from zenpy import Zenpy
from zenpy.lib.api_objects import CustomField

credentials = {
    'email': ***
    'token': ***
    'subdomain': ***,
}

zendesk = zenpy(**credentials)

ticket_50 = zendesk.tickets(id=50)
ticket_50.custom_fields.append(CustomField(id=123, value='Dummy'))
zendesk.tickets.update(ticket_50)

Am I doing something wrong? Or is not possible at all to achieve this?

Thank you!