How do I put a date into a custom field?

I'm trying to build a code where I need the field I created, "Created Time", and populate it with the current date whenever the ticket is created. Every time I run my code, however, I receive an error:

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

From my understanding, if I want to place somethign in a custom field, I need the API for it, which is what the error is calling an extra parameter, so I don't see what I'm missing. What do I need to do to fix this code and populate 'cf_created_date' with today's date?

Here's what I have:

month = today.getMonth();

day = today.getDay();

year = today.getYear();

Created_Date = "cf_created_date";

update = Map();

slash = "/";

newDate = month + slash + day + slash + year;

update.put(Created_Date,newDate);

response = zoho.desk.update(670765413,"tickets",ticketId,update);

info response;