<p>I'm trying to use a dojo combobox with an Ajax data source.  What I have is </p>

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

<p>Which does work except that I can't restrict the search set on the server side because I don't know how to change the url from which the data is pulled in order to specify a parameter.  Any hints? </p>

If I understand you correctly, you want the client to load different set of data from the server based on some general condition defined elsewhere.
Basically there is no need to have a
```
<div>
```
pre-defined. You can also create the
```
ItemFileReadStore
```
directly in JavaScript:
earlier...:
```
```
var tagMatchUrlBase = '<%=ResolveClientUrl("~/Tag/TagMatches")%>';
```
```
later...:
```
```
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
```
```