The End User API is now deprecated and has been replaced by the Customer API.
This endpoint gets or creates an end user. If the provided_id does not exist for the provided customer_id, a new end user will be created and information about the user will be sent back. If the provided_id does exist for the customer_id, the existing user will be returned.
A json mapping of key values you can use to load user-specific data into your
dashboards. Any key value is valid, but header and footer are reserved keys
that you can use to load html to display in the header and footer of your
email