namespace FCMClient
{
    [Service]
    [IntentFilter(new[] { "com.google.firebase.INSTANCE_ID_EVENT" })]
    public class MyFirebaseIIDService : FirebaseInstanceIdService
    {
        const string TAG = "MyFirebaseIIDService";
        public override void OnTokenRefresh()
        {
             var refreshedToken = FirebaseInstanceId.Instance.Token;
             Log.Debug(TAG, "Refreshed token: " + refreshedToken);
            SendRegistrationToServer(refreshedToken);
        }
        void SendRegistrationToServer(string token)
        {
            // Add custom implementation, as needed.
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
 package="com.clastech.testpush" android:installLocation="auto">
 <uses-sdk android:minSdkVersion="15" />
 <uses-permission android:name="android.permission.ACCESS_NOTIFICATION_POLICY" />
 <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

 <!--push notification-->
 <receiver
   android:name="com.google.firebase.iid.FirebaseInstanceIdInternalReceiver"
   android:exported="false" />
  <receiver
  android:name="com.google.firebase.iid.FirebaseInstanceIdReceiver"
  android:exported="true"
  android:permission="com.google.android.c2dm.permission.SEND">
  <intent-filter>
  <action android:name="com.google.android.c2dm.intent.RECEIVE" />
  <action android:name="com.google.android.c2dm.intent.REGISTRATION" />
  <category android:name="${applicationId}" />
  </intent-filter>
  </receiver>
  <!--push notification-->
   <application android:label="testePush.Android"></application>
 </manifest>