1. Ensure the you have set LaunchMode.SingleTop on your MainActivity:

LaunchMode.SingleTop

1     [Activity(Label = "云销管家",
2         Icon = "@mipmap/ic_launcher",
3         Theme = "@style/MainTheme",
4         LaunchMode = LaunchMode.SingleTop,
5         ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
6     public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity, IBDLocationListener
7     {

In your MainActivity (the FormsAppCompatActivity subclass) add a OnNewIntent override:

OnNewIntent:

1 protected override void OnNewIntent(Intent intent)
2 {
3     base.OnNewIntent(intent);
4     NotificationClickedOn(intent);
5 }

Now you can check the intent.Action / intent.HasExtra to determine if it is your notification that was send and thus process it. With Xamarin.Forms the easiest would be to use MessagingCenter to send a message that is subscribed to within your .NetStd/PCL Xamarin.Forms code base.

NotificationClickedOn:

 1         void NotificationClickedOn(Intent intent)
 2         {
 3 
 4             if (intent.Action == "LocalNotifierIntent1300" && intent.HasExtra("LocalNotification"))
 5             {
 6                 var notificationMessage = intent.Extras.GetString("LocalNotification");
 7                 var winnerToast = Toast.MakeText(this, $"{notificationMessage}.", ToastLength.Long);
 8                 winnerToast.SetGravity(Android.Views.GravityFlags.Center, 0, 0);
 9                 winnerToast.Show();
10             }
11         }

Send notification example:

 1 void SendNotifacation()
 2 {
 3     var title = "Winner, Winner, Chicken Dinner";
 4     var message = "You just won a million * reputation points";
 5 
 6     var intent = new Intent(BaseContext, typeof(MainActivity));
 7     intent.SetAction("ASushiNotification");
 8     intent.PutExtra("MessageFromSushiHangover", message);
 9     var pending = PendingIntent.GetActivity(BaseContext, 0, intent, PendingIntentFlags.CancelCurrent);
10 
11     using (var notificationManager = NotificationManager.FromContext(BaseContext))
12     {
13         Notification notification;
14         if (Android.OS.Build.VERSION.SdkInt < Android.OS.BuildVersionCodes.O)
15         {
16 #pragma warning disable CS0618 // Type or member is obsolete
17             notification = new Notification.Builder(BaseContext)
18                                                         .SetContentTitle(title)
19                                                         .SetContentText(message)
20                                                         .SetAutoCancel(true)
21                                                         .SetSmallIcon(Resource.Drawable.icon)
22                                                         .SetDefaults(NotificationDefaults.All)
23                                                         .SetContentIntent(pending)
24                                                         .Build();
25 #pragma warning restore CS0618 // Type or member is obsolete
26         }
27         else
28         {
29             var myUrgentChannel = BaseContext.PackageName;
30             const string channelName = "Messages from SushiHangover";
31 
32             NotificationChannel channel;
33             channel = notificationManager.GetNotificationChannel(myUrgentChannel);
34             if (channel == null)
35             {
36                 channel = new NotificationChannel(myUrgentChannel, channelName, NotificationImportance.High);
37                 channel.EnableVibration(true);
38                 channel.EnableLights(true);
39                 channel.SetSound(
40                     RingtoneManager.GetDefaultUri(RingtoneType.Notification),
41                     new AudioAttributes.Builder().SetUsage(AudioUsageKind.Notification).Build()
42                 );
43                 channel.LockscreenVisibility = NotificationVisibility.Public;
44                 notificationManager.CreateNotificationChannel(channel);
45             }
46             channel?.Dispose();
47 
48             notification = new Notification.Builder(BaseContext)
49                                                         .SetChannelId(myUrgentChannel)
50                                                         .SetContentTitle(title)
51                                                         .SetContentText(message)
52                                                         .SetAutoCancel(true)
53                                                         .SetSmallIcon(Resource.Drawable.icon)
54                                                         .SetContentIntent(pending)
55                                                         .Build();
56         }
57         notificationManager.Notify(1331, notification);
58         notification.Dispose();
59     }
60 }