I put this in my header and it works very well
<form action="products_search.php" method="GET" class="searchform" >
<input type="hidden" name="search_category_id" value="{top_id}">
<p>
<input class="search" type="text" name="search_string" value="{search_string}"/>
<input type="submit" name="search" class="button" value="Search" />
</p>
</form>
It was just hacked from the products_search.html