Thymeleaf dynamic form action does not work - spring

Question

I am loading the home.html page by setting the following model attributes:
#GetMapping("/")
private String getHomePage(Model model) {
model.addAttribute("allUsers", this.userService.findAll());
model.addAttribute("user", new User());
model.addAttribute("queryResult", "");
model.addAttribute("errors", new ArrayList<>());
return "home";
}
and here is my form for updating a user's data:
<form action="#" th:action="#{/user/{username}(username=${user.username})}" th:object="${user}" th:method="put">
<label for="username">Username</label>
<select th:field="*{username}" id="username">
<option th:each="singleUser : ${allUsers}" th:value="${singleUser.username}" th:text="${singleUser.username}"></option>
</select>
<label for="password">Password</label>
<input type="password" th:field="*{password}" id="password" >
<span th:if="${#fields.hasErrors('password')}" th:errors="*{password}"></span>
<label for="status">Status</label>
<select th:field="*{status}" id="status">
<option th:value="'Activated'" th:text="Activated"></option>
<option th:value="'Deactivated'" th:text="Deactivated"></option>
</select>
<button type="submit">UPDATE DATA</button>
</form>
When I try to submit this form, it does not bind the username to the URL.
instead of having /user/whateverUsername I have only /user.
I need to bind the value of this select element:
<select th:field="*{username}" id="username">
<option th:each="singleUser : ${allUsers}" th:value="${singleUser.username}" th:text="${singleUser.username}"></option>
</select>
How can I solve this problem?
Thank you very much!