code: function(params, callback) {
          /**
           * Callback for QB.pushnotifications.subscriptions.create(params, callback)
           * @callback createPushSubscriptionCallback
           * @param {object} error - The error object
           * @param {object} response - Array of all existent user's subscriptions
           */
          this.service.ajax({url: Utils.getUrl(config.urls.subscriptions), type: 'POST', data: params}, callback);
      }
docs: Create device based subscription (subscribes) ({@link https://docsdev.quickblox.com/rest_api/Push_Notifications_API.html#Create_subscription read more})
  @memberof QB.pushnotifications.subscriptions
  @param {object} params - Object of parameters
  @param {string} params.notification_channel - Declare which notification channels could be used to notify user about events. Allowed values: apns, apns_voip, gcm, mpns, bbps and email
  @param {object} params.push_token - Object of parameters
  @param {string} params.push_token.environment - Determine application mode. It allows conveniently separate development and production modes. Allowed values: evelopment or production
  @param {string} [params.push_token.bundle_identifier] - A unique identifier for client's application. In iOS, this is the Bundle Identifier. In Android - package id
  @param {string} params.push_token.client_identification_sequence - Identifies client device in 3-rd party service like APNS, GCM/FCM, BBPS or MPNS. Initially retrieved from 3-rd service and should be send to QuickBlox to let it send push notifications to the client
  @param {object} params.device - Object of parameters
  @param {string} params.device.platform - Platform of device, which is the source of application running. Allowed values: ios, android, windows_phone, blackberry
  @param {string} params.device.udid - UDID (Unique Device identifier) of device, which is the source of application running. This must be anything sequence which uniquely identify particular device. This is needed to support schema: 1 User - Multiple devices
  @param {createPushSubscriptionCallback} callback - The createPushSubscriptionCallback function