Memberstack custom fields
Sync Memberstack custom fields by creating a member with the email schema@whalesync.com and filling in a value for each custom field you want to sync.
Memberstack custom fields are supported differently in their API than other fields. In order to sync custom fields, you need to create a member in Memberstack with the email "schema@whalesync.com".
Once you create that member, add the custom fields you want to sync and make sure to put values in those fields.