XWalkWebViewEngine.java

//...

private void initWebViewSettings() {
  // ...
    String defaultUserAgent = webView.getUserAgentString();

    String overrideUserAgent = preferences.getString("OverrideUserAgent", null);
    if (overrideUserAgent != null) {
        webView.setUserAgentString(overrideUserAgent);
    } else {
        String appendUserAgent = preferences.getString("AppendUserAgent", null);
        if (appendUserAgent != null) {
            webView.setUserAgentString(defaultUserAgent + " " + appendUserAgent);
        }
    }
}

//...