Form action and Greybox

I' m using the following form on my website running on Drupal 6.1 :

<form id="searchForm" method="get" action="http://search-recherche.collectionscanada.ca/amicus/searchResults.jsp?Language=fre" target="greybox" >
        <p>
            <input type="hidden" name="FormName" value="AMICUS Simple Search" />
            <input type="hidden" name="PageNum" value="1" /> 
            <input type="hidden" name="SortSpec" value="score desc" />
            <input type="hidden" name="Language" value="fre" />
            <input type="hidden" name="QueryParser" value="lac_amicus" />
            <input type="hidden" name="Sources" value="amicus" />
            <input type="hidden" name="ResultCount" value="10" />           
           <input type="hidden" name="MaterialSource" value="" />
            <input type="hidden" name="MaterialDateOperator" value="" />                
            <input type="hidden" name="MaterialDate" value="" />    
        </p>
<p>
                    <select name="SearchIn"> 
                        <option value="">Tout mot-clé</option>
                        <option value="titles">Titre (mot-clé)</option> 
                        <option value="names">Nom (mot-clé)</option> 
                        <option value="subjects">Sujet (mot-clé)</option> 
                        <option value="namePart_main">Nom d'éditeur (mot-clé)</option>
                        <option value="notes">Notes (mot-clé)</option> 
                        <option value="isbn">ISBN</option> 
                        <option value="issn">ISSN</option>  
                        <option value="amicusNumber">Numéro AMICUS</option> 
                    </select>  
                <input type="text" name="SearchInText" value="" size="40" /><input type="submit" value="Cherchez" title="Cherchez" />
<br />
<a href="http://search-recherche.collectionscanada.ca/amicus/advSearch.jsp?Language=fre" title="Recherche élaborée" class="greybox">Recherche élaborée</a>&nbsp;&nbsp;<a href="http://www.collectionscanada.gc.ca/search/006001-130-f.html" title="Aide" class="greybox">Aide</a>
</p>
<legend><strong>Recherche dans :</strong>
                    <input type="radio" name="ParaUserView" value="" checked="checked" />Tout
                    <input type="radio" name="ParaUserView" value="UserView.Lac" />Catalogue de <acronym title="Biblioth&#232;que et Archives Canada">BAC</acronym>
                    <input type="radio" name="ParaUserView" value="UserView.Libraries" />Bibliothèques canadiennes
<br />
<span id="goto">
<strong>Autres recherches :</strong>&nbsp;
<a href="http://search-recherche.collectionscanada.ca/archives/search.jsp?Language=fre" title="Archives" class="greybox">Archives</a>,&nbsp;
<a href="http://search-recherche.collectionscanada.ca/ancestors/search.jsp?Language=fre" title="Ancêtres" class="greybox">Ancêtres</a>,&nbsp;
<a href="http://search-recherche.collectionscanada.ca/web/search.jsp?Language=fre" title="Site Web" class="greybox">Site Web de BAC</a>,&nbsp;
<a href="http://search-recherche.collectionscanada.ca/fed/search.jsp?Language=fre" title="Recherchez tout" class="greybox">Recherchez tout</a>&nbsp;
</span>
<br />
Pour plus d'option, consultez le <a href="http://amicus.collectionscanada.gc.ca/aaweb/aaloginf.htm" title="catalogue AMICUS" class="greybox">catalogue AMICUS</a>.
 </legend> 
 </form>

The search box perform perfectly and ofter the results in an new browser windows form Library and Archives Canada search pages. Howerver , how could I get the <form id="searchForm" method="get" action="http://search-recherche.collectionscanada.ca/amicus/searchResults.jsp?Language=fre" target="greybox" > attribute to open in a greybox windows?