The email template render request:

• EmailTemplateId (string, optional): The ID of the email template. Guid Id.
• RecipientType (string, optional): Type of recipient.
• RecipientId (string, optional): ID of the recipient. Guid Id.
• ObjectName (string, optional):DataBase object name.
• RecordId (string, optional): ID of the record. Guid Id.
• TemplateData (object, optional): Key-value pairs for template data.
  Format: { key: value, ... }
• RecordDetails (object, optional): Key-value pairs for record details.
  Format: { key: value, ... }
• Subject (string, optional): Email subject line.
• HtmlBody (string, optional): HTML content of the email.
• PlainTextBody (string, optional): Plain text content of the email.