code: public Integer postCharactersCharacterIdMailLabels(Integer characterId, String datasource, String token,
              MailLabelSimple mailLabelSimple) throws ApiException {
          ApiResponse<Integer> resp = postCharactersCharacterIdMailLabelsWithHttpInfo(characterId, datasource, token,
                  mailLabelSimple);
          return resp.getData();
      }
docs: Create a mail label Create a mail label --- SSO Scope:
  esi-mail.organize_mail.v1
  
  @param characterId
  An EVE character ID (required)
  @param datasource
  The server name you would like data from (optional, default to
  tranquility)
  @param token
  Access token to use if unable to set a header (optional)
  @param mailLabelSimple
  (optional)
  @return Integer
  @throws ApiException
  If fail to call the API, e.g. server error or cannot
  deserialize the response body