Cannot deserialize instance of `java.lang.Long` out of START_OBJECT token; on Spring boot post

I want to post a new Game to my spring boot backend from my angular application. I do this through an post from angular. This game has a relationship with my user entity in spring boot JPA. But when I do my post I get an 400 error code in the console which says:

"MethodArgumentNotValidException","fieldErrors":[{"field":"user","errorCode":"NotNull"}]}

This was the data from my post:

{category: "sport", sessieId: 20004, userID: 10004, score: null}

My game controller:

@CrossOrigin(origins = "*", allowedHeaders = "*")
@RestController
@RequestMapping(value = "/api/games", produces = MediaType.APPLICATION_JSON_VALUE)
public class GameController {

@PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public Long createGame(@RequestBody @Valid final GameDTO gameDTO) {
        return gameService.create(gameDTO);
    }
 // more methods for http
}

my Game domain:

@Entity
public class Game {

    @Id
    @Column(nullable = false, updatable = false)
    @SequenceGenerator(name = "primary_sequence", sequenceName = "primary_sequence",
            allocationSize = 1, initialValue = 10000)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "primary_sequence")
    private Long id;

    @Column(nullable = false)
    private Integer sessieId;

    @Column(nullable = false)
    private String category;

    @Column
    private String score;

    @ManyToOne
    @JoinColumn(name = "user_id", nullable = false)
    private User user;

my Game DTO:

public class GameDTO {

    private Long id;

    @NotNull
    private Integer sessieId;

    @NotNull
    @Size(max = 255)
    private String category;

    @Size(max = 255)
    private String score;

    @NotNull
    private Long user;

The create method from my Game service:

public Long create(final GameDTO gameDTO) {
        final Game game = new Game();
        mapToEntity(gameDTO, game);
        return gameRepository.save(game).getId();
    }

So I saw the Game entity expects a long from user, so I tried to post the object user in the post for game. the post body:

{category: "sport", sessieId: 20004,…}
category: "sport"
score: null
sessieId: 20004
user: {id: 10004, firstName: "test5", lastName: "test5", email: "test5@test.nl", password: "test"}

But then I got the following error:

JSON parse error: Cannot deserialize instance of java.lang.Long out of START_OBJECT token;

Do I need to pass in a user object on the post? and how can I then fix the error?