maks was trying2

src/main/resources/templates/product-filter.html

@@ -36,7 +36,7 @@
       </tbody>
     </table>
   </div>
-  <form th:action="@{/product/filter}" method="post">
+  <form th:action="@{/product/filter}" th:object="${selmanufacturers}" method="post">
     <div class="input-group mb-3">
       <select class="form-select" th:name="manufacturerid">
         <option th:each="manufacturer, iterator: ${manufacturers}" th:value="${manufacturer.id}" th:text="${manufacturer.name}"></option>