Custom message in HTML format in email body

I am writing you in regard of Echo sign API version v5, Echo sign API support sending of custom message’s in email body. We are looking for option to pass message in HTML format so formatting can be applied on custom message (Tabular formatting). Request you to please suggest how to achieve this via API integration.

Here is the Adobe documentation I followed:

Api Version : v5

API: https://www.adobe.io/apis/documentcloud/sign/docs.html#!adobedocs/adobe-sign/master/api_usage/send_signing.md

Custom message param: “Message” property in documentCreationInfo.

  As of now above implementation not supporting HTML format.