code: public Account setCallback(String callback) throws HelloSignException {
          return new Account(httpClient.withAuth(auth).withPostField(Account.ACCOUNT_CALLBACK_URL, callback)
                  .post(BASE_URI + ACCOUNT_URI).asJson());
      }
docs: Update your account callback URL.
  
  This URL is used to notify you of any signature request events that occur
  when your account is a party -- e.g., sender or signer/recipient.
  
  @param callback String URL
  @return Account
  @throws HelloSignException thrown if there's a problem processing the
  HTTP request or the JSON response.