code: public String add_users_list(Map<String, Object> data) {
          String id = data.get("id").toString();
          Gson gson = new Gson();
          String json = gson.toJson(data);
          return post("list/" + id + "/users", json);
      }
docs: /*
  Add already existing users in the SendinBlue contacts to the list.
  @param {Object} data contains json objects as a key value pair from HashMap.
  @options data {Integer} id: Id of list to link users in it [Mandatory]
  @options data {Array} users: Email address of the already existing user(s) in the SendinBlue contacts. Example: "test@example.net". You can use commas to separate multiple users [Mandatory]