Below is the code to add to your custom page template in order to place the ShopSite search field on your ShopSite generated page.
<form action="[-- SHOPPING_CART_URL BASE --]/productsearch.cgi?storeid=[-- STORE.ID --]" method="post">
<input type=hidden name="storeid" value="[-- STORE.ID --]">
<input type="text" name="search_field" size="13" value="[-- STORE.Search --]" onFocus="value=''">
<input type="submit" value="[-- STORE.Go --]">
</form>