code: private String executeScriptExecuteRequest(String rScript) throws IOException {
      URI uri = getScriptExecutionUri();
      HttpPost httpPost = new HttpPost(uri);
      NameValuePair nameValuePair = new BasicNameValuePair("x", rScript);
      httpPost.setEntity(new UrlEncodedFormEntity(singletonList(nameValuePair)));
  
      String openCpuSessionKey;
      try (CloseableHttpResponse response = httpClient.execute(httpPost)) {
        int statusCode = response.getStatusLine().getStatusCode();
        if (statusCode >= 200 && statusCode < 300) {
          Header openCpuSessionKeyHeader = response.getFirstHeader("X-ocpu-session");
          if (openCpuSessionKeyHeader == null) {
            throw new IOException("Missing 'X-ocpu-session' header");
          }
          openCpuSessionKey = openCpuSessionKeyHeader.getValue();
          EntityUtils.consume(response.getEntity());
        } else if (statusCode == 400) {
          HttpEntity entity = response.getEntity();
          String rErrorMessage = EntityUtils.toString(entity);
          EntityUtils.consume(entity);
          throw new ScriptException(rErrorMessage);
        } else {
          throw new ClientProtocolException(format(FORMAT_UNEXPECTED_RESPONSE_STATUS, statusCode));
        }
      }
      return openCpuSessionKey;
    }
docs: Execute R script using OpenCPU
  
  @param rScript R script
  @return OpenCPU session key
  @throws IOException if error occured during script execution request