How posting One Signal notification's additional data and receiving that?

You set the data field as a key in the dictionary passed to OneSignal.postNotification like the following.

OneSignal.postNotification(["contents": ["en": "Test Message"],
                            "include_player_ids": ["3009e210-3166-11e5-bc1b-db44eb02b120"],
                            "data": ["postID": "id"]])

Then you need to get ready your keys from additionalData from the payload in the handleNotificationAction function.

if let additionalData = payload?.additionalData {
   let postID: String? = additionalData["postID"]
}

Example from iOS in objC to send additional data...

[OneSignal postNotification:@{@"contents":@{@"en":text},
                              @"include_player_ids":oneSignalIds,
                              @"data":@{@"key": @"value"},
                              }];

And to receive the data...

 [OneSignal initWithLaunchOptions:launchOptions
                           appId:ONESIGNAL_APPID
      handleNotificationReceived:^(OSNotification *notification) {

          if (notification.payload.additionalData) {

              NSDictionary* additionalData = notification.payload.additionalData;

              if (additionalData[@"key"]){
                  NSLog(@"Received Data - %@", additionalData[@"key"]);
              }
          }
      }

        handleNotificationAction:nil
                        settings:@{kOSSettingsKeyInAppAlerts:@YES}];

Hope it helps someone :)