gistfile1.txt

  public static void main(String[] args) throws MalformedURLException {
    MutableCapabilities caps = new FirefoxOptions().setHeadless(false).setLegacy(false);
    Map<String, Object> saasOptions = ImmutableMap.of(
        "screenResolution", "1280x768",
        "platform", "Windows 10",
        "version", "57.0");
    caps.setCapability("saas:config", saasOptions);

    WebDriver driver = new RemoteWebDriver(
        new URL("http://cloud.example.com"),
        caps);
    driver.quit();
  }

JSON output of caps is:

{
  "acceptInsecureCerts": true,
  "browserName": "firefox",
  "marionette": true,
  "moz:firefoxOptions": {
    "args": [
      
    ],
    "prefs": {
      
    }
  },
  "saas:config": {
    "screenResolution": "1280x768",
    "platform": "Windows 10",
    "version": "57.0"
  }
}