You should open your Nabble application and look at the HTML source. Then search for "search-box" and you will find a HTML form element. For example, the code below belongs to the Nabble Support forum:
<span id="search-box" style="clear:both">
<form style="margin-top:.3em" action="/template/NamlServlet.jtp">
<input type="hidden" name="macro" value="search_page" />
<input type="hidden" name="node" value="5621697" />
<table>
<tr valign="top">
<td align="right">
<input name="query" size="20" />
</td>
<td>
<input type="submit" value="Search" class="go-btn" />
</td>
<td align="center">
<a href="/template/NamlServlet.jtp?macro=adv_search_page&node=5621697" rel="nofollow" style="font-size:70%">Advanced<br/>Search</a>
</td>
</tr>
</table>
</form>
</span>
You would have to make a small change to the code above. You have to add your app domain (e.g., my-app.n2.nabble.com) to the URLs (i.e., form action and advanced search link).