code: public StatusCode modifyWatchList(String sessionId, int accountId, MediaType mediaType, Integer movieId, boolean addToWatchlist) throws MovieDbException {
          TmdbParameters parameters = new TmdbParameters();
          parameters.add(Param.SESSION_ID, sessionId);
          parameters.add(Param.ID, accountId);
  
          String jsonBody = new PostTools()
                  .add(PostBody.MEDIA_TYPE, mediaType.toString().toLowerCase())
                  .add(PostBody.MEDIA_ID, movieId)
                  .add(PostBody.WATCHLIST, addToWatchlist)
                  .build();
  
          URL url = new ApiUrl(apiKey, MethodBase.ACCOUNT).subMethod(MethodSub.WATCHLIST).buildUrl(parameters);
          String webpage = httpTools.postRequest(url, jsonBody);
  
          try {
              return MAPPER.readValue(webpage, StatusCode.class);
          } catch (IOException ex) {
              throw new MovieDbException(ApiExceptionType.MAPPING_FAILED, "Failed to modify watch list", url, ex);
          }
      }
docs: Add or remove a movie to an accounts watch list.
  
  @param sessionId
  @param accountId
  @param movieId
  @param mediaType
  @param addToWatchlist
  @return
  @throws MovieDbException