Setting entity reference by html selector

Good morning, I have a problem with adding new entity with reference to another entity by html selector. Generally it works for String and int value but not for another object. I use this html:

<form action="#" th:action="@{/addTripCommentPost}" th:object="${tripComment}"
        method="POST">
    <input type="text" th:field="*{comment}"
           placeholder="Wpisz komentarz">
    <select th:field="*{trip}">
      <option th:each="tripTH : ${trips}"
              th:value="${tripTH}"
              th:text="${tripTH.getName()}"/>
    </select>
    <input type="submit" value="Dodaj">
</form>

And for the java code: @Controller public class TripController {

@Autowired
private TripService tripService;

@GetMapping("/add-trip")
public String addTrip(Model model) {
    Trip trip = new Trip();
    model.addAttribute("trip", trip);
    return "add-trip";
}

@PostMapping("/addTripPost")
public String addTripPost(@ModelAttribute("trip") Trip trip) {
    tripService.addTrip(trip);
    return "redirect:/";
}
}

@Entity
public class TripComment {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

private String comment;
@ManyToOne(optional = false)
@JoinColumn(name = "trip_id", nullable = false)
private Trip trip;

@ManyToOne(optional = false)
@JoinColumn(name = "appUser_id", nullable = false)
private AppUser appUser;

private String nameOfTrip;
}

And I reveived this error:

2022-09-13 21:40:23.985  WARN 16892 --- [nio-8080-exec-8] .w.s.m.s.DefaultHandlerExceptionResolver : 
Resolved [org.springframework.validation.BindException: org.springframework.validation.BeanPropertyBindingResult: 
1 errors<EOL>Field error in object 'tripComment' on field 'trip': 
rejected value [Trip{name='Tirol Prestige #1', country=AUSTRIA, price=2500.0, link='xxx', startDate=2023-03-10, finishDate=2023-03-10}]; 
codes [typeMismatch.tripComment.trip,typeMismatch.trip,typeMismatch.com.example.skiapp.trip.Trip,typeMismatch]; 
arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [tripComment.trip,trip]; 
arguments []; default message [trip]]; default message [Failed to convert property value of type 'java.lang.String' 
to required type 'com.example.skiapp.trip.Trip' for property 'trip'; nested
exception is org.springframework.core.convert.ConversionFailedException:
Failed to convert from type [java.lang.String] to type [java.lang.Long] for value 
'Trip{name='Tirol Prestige #1', country=AUSTRIA, price=2500.0, link='xxx', startDate=2023-03-10, finishDate=2023-03-10}'; 
nested exception is java.lang.NumberFormatException: For input string: 
"Trip{name='TirolPrestige#1',country=AUSTRIA,price=2500.0,link='xxx',startDate=2023-03-10,finishDate=2023-03-10}"]]

Do you know what is wrong and how to fix it? Temporialy I am using a String value to catch the name and nextly I convert it in controller but if it possible I prefer to know what is wrong.