How to send POST request with Many to many relationship in Spring Boot - spring

Question

Anyone have any ideas on how I could do postmapping for the Many-to-Many relationship? Getting the data works, but this is what I'm having trouble with
I tried using the "guide" but unfortunately I don't understand it very well yet
Here is my entities:
Album
#Entity
public class Album implements Serializable {
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
Long id;
String name;
String artist;
String cover;
#ManyToMany(fetch = FetchType.LAZY, cascade =
{
CascadeType.DETACH,
CascadeType.MERGE,
CascadeType.REFRESH,
CascadeType.PERSIST
})
#JoinTable( name = "user_albums",
joinColumns = #JoinColumn(name = "album_id", nullable = false),
inverseJoinColumns = #JoinColumn(name = "user_id", nullable = false)
)
#JsonBackReference
#OnDelete(action = OnDeleteAction.CASCADE)
private Set<User> users = new HashSet<>();
User:
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#NotBlank
#Size(max = 20)
private String username;
#NotBlank
#Size(max = 50)
#Email
private String email;
#NotBlank
#Size(max = 120)
private String password;
#ManyToMany(fetch = FetchType.LAZY, cascade =
{
CascadeType.DETACH,
CascadeType.MERGE,
CascadeType.REFRESH,
CascadeType.PERSIST
})
#JoinTable( name = "user_albums",
joinColumns = #JoinColumn(name = "user_id", nullable = false),
inverseJoinColumns = #JoinColumn(name = "album_id", nullable = false)
)
#OnDelete(action = OnDeleteAction.CASCADE)
private Set<Album> albums = new HashSet<>();
I tried it this way but it didn't work
#PostMapping("/users/mal/{userId}/album")
public ResponseEntity<Album> addAlbum(#PathVariable(value = "userId") Long userId, #RequestBody Album albumRequest, User userRequest) {
Album newMal = userRepo.findById(userId).map(user -> {
long albumId = userRequest.getId();
user.addAlbum(albumRequest);
return albumRepo.save(albumRequest);
}).orElseThrow(() -> new RuntimeException("Not found USER with id = " + userId));
return new ResponseEntity<>(newMal, HttpStatus.CREATED);