This is for the MD5 hash of the lowercase version of the list member’s email address, which is how you would update an existing member in your list. More information on this can be found in the Mailchimp documentation.
Wow! Tom, have you received any info about it? we are trying to do pretty much the same thing you were trying to accomplish but I can't figured out how to add the necessary fields.
We recommend opening a ticket so Support can review your configuration and work with you on a solution. Thanks!
I did that Tom. Thanks!