NotificationPayload received in the callback is a MoEPushCampaign instance with the following definition:

class MoEPushCampaign {
  platform: String;
  payload: Map<String, Object>;
  isDefaultAction: Boolean
  clickAction: Map<String, Object>
}

Payload Structure for clickedAction Map

{
  "clickedAction": {
    "type": "navigation/customAction",
    "payload": {
      "type": "screenName/deepLink/richLanding",
      "value": "",
      "kvPair": {
        "key1": "value1",
        "key2": "value2",
        ...
      }
    }
  }
}

platform - Native platform from which callback is triggered. Possible values - android, ios
isDefaultAction - This key is present only for the Android Platform. It's a boolean value indicating if the user clicked on the default content or not. true if the user clicks on the default content else false.
clickedAction - Action to be performed on notification click.
clickedAction.type - Type of click action. Possible values navigation and customAction. Currently, customAction is supported only on Android.
clickAction.payload - Action payload for the clicked action.
clickedAction.payload.type - Type of navigation action defined. Possible values screenName, deepLink, richLanding. Currently, in the case of iOS, richlanding and deep-link URL are processed internally by the SDK and not passed in this callback therefore possible value in case of iOS is only screenName.
clickAction.value - value entered for navigation action or custom payload.
clickAction.kvPair - Custom key-value pair entered on the MoEngage Platform.
payload - Complete campaign payload.

Android Payload

If the user clicks on the default content of the notification the key-value pair and campaign payload can be found inside the payload key. If the user clicks on the action button or a push template action the action payload would be found inside clickedAction.
You can use the isDefaultAction key to check whether the user clicked on the default content or not and then parse the payload accordingly.

iOS Payload

In the case of iOS, you would always receive the key-value pairs with respect to clicked action in clickedAction property. Refer to this Notification Payload for knowing the iOS notification payload structure.

Notification Click Observers:

MoEngage plugin triggers the pushClicked event whenever a notification is clicked. This event is a common trigger for both iOS and Android platforms and is available from plugin version 6.0.0. Refer to the below code to set listener to the same:

import ReactMoE from 'react-native-moengage' 
ReactMoE.setEventListener("pushClicked", (notificationPayload) => { 
  console.log("pushClicked", notificationPayload); 
});

Make sure this callback is set as soon as the application is initialized. Preferably in the constructor of your App.js.