It would function very similar to the "Unsubscribe" function Zoho provides, but instead of updating the "Email Opt-Out" flag for the given email, it would update a particular field for a given contactId and a value for that field (passed as 2 parameters).

This sounds simple enough, but with no experience with the Zoho API I'm at a loss where even to begin.  I have implemented this functionality as a Zoho Creator App with just 2 function calls, but Zoho Creator Apps seem to need a log-in to be run (therefore unsuitable for what I'm trying to accomplish).

Could someone help me with such a script?

Thanks.

Oliver