This method is used to add Subscription Documents to the offering through DocuSign. A template can be set up in DocuSign, allowing investor data to be auto-populated when the documents are sent. The Offering ID is required to add a Template Document. This method will return the Template ID as a successful response. To obtain the templateId needed for the templatename param, fetchSubscriptionDocuments can be used. A DocuSign business account connected to TransactAPI is required to use this method.