where the update data is a dict of properties i want to update/add. But I don't know what the acceptable property name of additional email is through the api. I've tried 'additional_email', 'hs_additional_email', 'secondary_email'. None of them seem to work.
The correct property name for additional email address(es) belonging to a Contact is "hs_additional_emails". However, it is set by HubSpot as "read only". There are some legacy API endpoints that you might consider using (see below). Note that you may want to consider using Python's "requests" module to interact with those legacy APIs, as HubSpot's Python Client will likely not offer support for them.
The correct property name for additional email address(es) belonging to a Contact is "hs_additional_emails". However, it is set by HubSpot as "read only". There are some legacy API endpoints that you might consider using (see below). Note that you may want to consider using Python's "requests" module to interact with those legacy APIs, as HubSpot's Python Client will likely not offer support for them.