Can't create contact in ticket with custom fields(cf)

Can't create contact in ticket with custom fields(cf)

I'm trying to create a ticket with a contact that contains a custom field, but I can't .

{
    
    "departmentId": xxxx,
    "subject": "Prueba",
    "description": "Description",
    "priority": "High",
    "category": "Others",
    "classification": "Problem",
    "dueDate": "2021-08-12T13:00:00Z",
    "contact": 
    {"firstName": "Name",
    "lastName": "LastName",
    "email": "xxxxxx",
    "cf": 
    {
        "cf_type_document": "C.C",
        "cf_id_document": "1037662294"
    }
    }
}

Response:
{
    "errorCode": "UNPROCESSABLE_ENTITY",
    "message": "An extra parameter 'cf' is found"
}