Dynamic url for dojo combobox source

<div dojoType="dojo.data.ItemFileReadStore"
     jsId="tags"
     url="<%=ResolveClientUrl("~/Tag/TagMatches")%>" >
</div>
<select dojoType="dijit.form.ComboBox"
        store="tags"
        value=""
        name="tagName">
</select>

var tagMatchUrlBase = '<%=ResolveClientUrl("~/Tag/TagMatches")%>';

var tagMatchUrl = tagMatchUrlBase + "?f=" + escape(somefilterString);
var store = new dojo.data.ItemFileReadStore({url: tagMatchUrl});
tagName.store = store;
// maybe use store.fetch() to pre-select item #1