Example how to monetize yours Xamarin.Forms applications with AdMob by Google - http://julianocustodio.com/admob-xamarin-forms

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="1" android:versionName="1.0" package="com.companyname.DemoAdMob" android:installLocation="auto">
	<uses-sdk android:minSdkVersion="15" />
	<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
	<uses-permission android:name="android.permission.INTERNET" />
	<application android:label="DemoAdMob.Android"></application>

  <activity android:name="com.google.android.gms.ads.AdActivity" android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize" android:theme="@android:style/Theme.Translucent" />
</manifest>