code: public Template addTemplateUser(String templateId, String idOrEmail) throws HelloSignException {
          String url = BASE_URI + TEMPLATE_ADD_USER_URI + "/" + templateId;
          String key = (idOrEmail != null && idOrEmail.contains("@")) ? Account.ACCOUNT_EMAIL_ADDRESS
                  : Account.ACCOUNT_ID;
          return new Template(httpClient.withAuth(auth).withPostField(key, idOrEmail).post(url).asJson());
      }
docs: Adds the provided user to the template indicated by the provided template
  ID. The new user can be designated using their account ID or email
  address.
  
  @param templateId String template ID
  @param idOrEmail String account ID or email address
  @return Template
  @throws HelloSignException thrown if there's a problem processing the
  HTTP request or the JSON response.