as on Android, you should use an Intent Filter to achieve that functionnality, but these are implemented as class attributes in Xamarin.Android, that will be automatically translated to xml in the app manifest at compile time for you.
here's an example to get you going :
[Activity (Label = "MainActivity", MainLauncher = true)]
[IntentFilter (new[]{Intent.ActionMain},
Categories=new[]{Intent.CategoryLauncher, Intent.CategorySampleCode},
Icon="@drawable/myicon",
DataScheme="something",
DataHost="project.example.com")]
public class MainActivity : Activity
{
}
you can get more details on IntentFilters in the Xamarin docs
watch out if you copy paste their sample, it misses a comma there. so you're no searching for too long like I did ;-)