Firebase: How to set default notification channel in Android app?

You need to have registered a channel using NotificationManager CreateNotificationChannel.

This example uses c# in Xamarin, but is broadly applicable elsewhere

private void ConfigureNotificationChannel()
{
    if (Build.VERSION.SdkInt < BuildVersionCodes.O)
    {
        // Notification channels are new in API 26 (and not a part of the
        // support library). There is no need to create a notification
        // channel on older versions of Android.
        return;
    }

    var channelId = Resources.GetString(Resource.String.default_notification_channel_id);
    var channelName = "Urgent";
    var importance = NotificationImportance.High;

    //This is the default channel and also appears in the manifest
    var chan = new NotificationChannel(channelId, channelName, importance);

    chan.EnableVibration(true);
    chan.LockscreenVisibility = NotificationVisibility.Public;

    var notificationManager = (NotificationManager)GetSystemService(NotificationService);
    notificationManager.CreateNotificationChannel(chan);

}

This channel should be unique eg com.mycompany.myapp.urgent

You then add a reference to a string inside the application tag in the AndroidManifest.xml

<application android:label="MyApp" android:icon="@mipmap/icon">
    <meta-data android:name="com.google.firebase.messaging.default_notification_channel_id" android:value="@string/default_notification_channel_id" />
</application>

Finally, setup the string in strings.xml

<?xml version="1.0" encoding="UTF-8" ?>
<resources>
    <string name="default_notification_channel_id">com.mycompany.myapp.urgent</string>
</resources>

As you can see here in the official docs, you need to add the following metadata element in your AndroidManifest.xml within the application component:

<meta-data
    android:name="com.google.firebase.messaging.default_notification_channel_id"
    android:value="@string/default_notification_channel_id"/>

This default channel will be used when notification message has no specified channel, or if the channel provided has not yet been created by the app.