Firebase Cloud Messaging (FCM) - Launch Activity when user clicks the notification with extras

Ok, I have found the solution.

This is the json that I'm sending from the server to the app

{
  "registration_ids": [
    "XXX",
    ...
  ],
  "data": {
    "id_offer": "41"
  },
  "notification": {
    "title": "This is the Title",
    "text": "Hello I'm a notification",
    "icon": "ic_push",
    "click_action": "ACTIVITY_XPTO"
  }
}

At the AndroidManifest.xml

<activity
    android:name=".ActivityXPTO"
    android:screenOrientation="sensor"
    android:windowSoftInputMode="stateHidden">
    <intent-filter>
        <action android:name="ACTIVITY_XPTO" />        
        <category android:name="android.intent.category.DEFAULT" />
    </intent-filter>
</activity>

When the app is closed or in background and the user clicks on the notification it opens my ActivityXPTO, to retrieve the id_offer I only need to do

public class ActivityXPTO extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        ...

        String idOffer = "";

        Intent startingIntent = getIntent();
        if (startingIntent != null) {
            idOffer = startingIntent.getStringExtra("id_offer"); // Retrieve the id
        }

        getOfferDetails(idOffer);
    }

    ...
}

That's it...