Need Help on Http Header to add my own User-Agent with CDHTMLDialog

I use CDHTMLDialog to program a simple web browser, and I had a trobule
on Http Header modify.
I want add my own "User-Agent" when client use my application to browse
web.

//------------------------------------------------------------------------------------------------
#define WM_NVTO (WM_USER+1000)

class NvToParam
{
public:
    CString URL;
    DWORD Flags;
    CString TargetFrameName;
    CByteArray PostedData;
    CString Headers;
    DWORD dwPostDataLen;
};
//------------------------------------------------------------------------------------------------
LRESULT Ccdhtml_t1Dlg::OnNvTo(WPARAM wParam, LPARAM lParam)
{
    NvToParam* pNvTo = (NvToParam*)wParam;

    CDHtmlDialog::Navigate((LPCTSTR)pNvTo->URL, pNvTo->Flags,
(LPCTSTR)pNvTo->TargetFrameName, (LPCTSTR)pNvTo->Headers,
pNvTo->PostedData.GetData());

    delete pNvTo;
    return 1;
}

//------------------------------------------------------------------------------------------------
void Ccdhtml_t1Dlg::OnBeforeNavigate2(LPDISPATCH pDisp, VARIANT FAR*
URL, VARIANT FAR* Flags, VARIANT FAR* TargetFrameName, VARIANT FAR*
PostData, VARIANT FAR* Headers, BOOL FAR* Cancel)
{
        CString strHeaders(V_BSTR(Headers));

    // Check User-Agent
    if(strHeaders.Find("User-Agent:LHPBrowser 1.0") < 0)
        {
            *Cancel = TRUE; // Cancel this request

            if(!strHeaders.IsEmpty())
                strHeaders += "\r\n";
        // Add Customize Http Header
            strHeaders += "User-Agent:LHPBrowser 1.0";

            NvToParam* pNvTo = new NvToParam;
            ASSERT(V_VT(URL) == VT_BSTR);
            ASSERT(V_VT(TargetFrameName) == VT_BSTR);
            ASSERT(V_VT(PostData) == (VT_VARIANT | VT_BYREF));
            ASSERT(V_VT(Headers) == VT_BSTR);
            ASSERT(Cancel != NULL);

            USES_CONVERSION;

            VARIANT* vtPostedData = V_VARIANTREF(PostData);
            CByteArray array;
            if (V_VT(vtPostedData) & VT_ARRAY)
            {
                // must be a vector of bytes
                ASSERT(vtPostedData->parray->cDims == 1 &&
vtPostedData->parray->cbElements == 1);

                vtPostedData->vt |= VT_UI1;
                COleSafeArray safe(vtPostedData);

                DWORD dwSize = safe.GetOneDimSize();

                LPVOID pVoid;
                safe.AccessData(&pVoid);

                array.SetSize(dwSize);
                LPBYTE lpByte = array.GetData();

                memcpy(lpByte, pVoid, dwSize);

                safe.UnaccessData();
            }