How to execute a command in a XUL element?

From the navigator.xul,
            <textbox id="urlbar" class="plain" flex="1"
                     type="autocomplete" searchSessions="history"
                     timeout="50" maxrows="6" alwaysOpenPopup="true"
                     defaultSearchEngine="true" tabScrolling="true"
                     showCommentColumn="true"
                     tooltip="aTooltip" tooltiptext="&locationBar.tooltip;"
                     ontextcommand="return handleURLBarCommand(userAction);"
                     ontextrevert="return handleURLBarRevert();"
                     onfocus="URLBarFocusHandler(event);"
                     onblur="URLBarBlurHandler(event);">
              <image id="page-proxy-button" allowevents="true"
                     ondraggesture="PageProxyDragGesture(event);"/>
              <menupopup id="ubhist-popup"
class="autocomplete-history-popup"
                         popupalign="topleft" popupanchor="bottomleft"
                         onpopupshowing="createUBHistoryMenu(event.target);"

oncommand="executeUrlBarHistoryCommand(event.target);"/>
            </textbox>

I can set the value of the URL location bar to particular URL using the
following code
    nsCOMPtr<nsIDOMDocument> pDoc;
    res = m_TopDomWin->GetDocument(getter_AddRefs(pDoc));
            nsAutoString szTag;
            szTag.AssignWithConversion("urlbar");
  nsCOMPtr<nsIDOMElement> pUrlBar;
  res = pDoc->GetElementById(szTag, getter_AddRefs(pUrlBar));

  nsAutoString szValue;
            szValue.AssignWithConversion("value");
  res = pUrlBar->SetAttribute(szValue, sUrl);

However, I cannot navigate the browser to that location.
Is there anyway I can execute the ontextcommand?

Thanks

Jacob Wong