How to implement Find-on-Page command in MFC web browser control with pre-filled search word?

I'm using the MFC web browser control in my dialog-based MFC project to display some HTML content and I am trying to make it show the "Find on page" dialog window with the search word already pre-selected in it. (What you'd get if you hit Ctrl+F in the IE web browser.)

If I do the following the find dialog is shown OK, but it doesn't seem to support a way to pre-fill the search word:

IWebBrowser2* pWebBrowser = NULL;
LPUNKNOWN unknown = m_browser.GetControlUnknown();

unknown->QueryInterface(IID_IWebBrowser2,(void **)&pWebBrowser);

if(pWebBrowser)
{
    HRESULT hr;
    CComVariant varNull;
    if(SUCCEEDED(hr = pWebBrowser->ExecWB(OLECMDID_FIND, OLECMDEXECOPT_PROMPTUSER, &varNull, &varNull)))
    {
        //Success!
    }

    pWebBrowser->Release();
}

if(unknown)
{
    unknown->Release();
}

I found this MSDN page, that says:

OLECMDID_SHOWFIND Tells the receiver to show the Find dialog box. It takes a VT_DISPATCH input param.

So evidently there's another Find command with OLECMDID_SHOWFIND ID, but I can't seem to make it work. I don't understand what is this VT_DISPATCH input param either? When I try this -- a total shot in the dark due to the lack of documentation:

CComVariant var1 = L"Pre-filled search word 1", var2 = L"Pre-filled search word 2";
hr = pWebBrowser->ExecWB(OLECMDID_SHOWFIND, OLECMDEXECOPT_DODEFAULT, &var1, &var2);

but I get hr=0x80040100 or "Trying to revoke a drop target that has not been registered"

Any idea how to make it work?