Example of Push Notifications with OneSignal in Xamarin.Forms applications - http://julianocustodio.com/onesignal-push-notification

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="{InsertPackageNameHere}" android:installLocation="auto">
	<uses-sdk android:minSdkVersion="15" />
  
  <permission android:name="{InsertPackageNameHere}.permission.C2D_MESSAGE" android:protectionLevel="signature" />
  <uses-permission android:name="{InsertPackageNameHere}.permission.C2D_MESSAGE" />
	
  <application android:label="DemoNotification.Android">
    <receiver android:name="com.onesignal.GcmBroadcastReceiver" android:permission="com.google.android.c2dm.permission.SEND">
      <intent-filter>
        <action android:name="com.google.android.c2dm.intent.RECEIVE" />
        <category android:name="{InsertPackageNameHere}" />
      </intent-filter>
    </receiver>
  </application>
  
</manifest>