MainActivity.cs

using System;
using Android.App;
using Android.Widget;
using Android.OS;
using Android.Content;
using Android.Content.PM;

namespace XamarinWhatsapp
{
  [Activity(Label = "XamarinWhatsapp", MainLauncher = true, Icon = "@drawable/icon")]
  public class MainActivity : Activity
  {
    Button btnEnviar;
    EditText edtMensaje;
    string whatsapp = "com.whatsapp";

    bool VerificarApp(String uri)
    {
      try
      {
        ApplicationContext.PackageManager.GetPackageInfo(uri, PackageInfoFlags.Activities);
        return true;
      }
      catch (PackageManager.NameNotFoundException e)
      {
        return false;
      }
    }

    protected override void OnCreate(Bundle bundle)
    {
      base.OnCreate(bundle);
      SetContentView(Resource.Layout.Main);
      btnEnviar = FindViewById<Button>(Resource.Id.btnEnviar);
      edtMensaje = FindViewById<EditText>(Resource.Id.edtMensaje);
      btnEnviar.Click += BtnEnviar_Click;
    }

    private void BtnEnviar_Click(object sender, System.EventArgs e)
    {
      if (VerificarApp(whatsapp))
      {
        Intent intent = new Intent();
        intent.SetAction(Intent.ActionSend);
        intent.PutExtra(Intent.ExtraText, edtMensaje.Text);
        intent.SetType("text/plain");
        intent.SetPackage(whatsapp);
        StartActivity(intent);
      }
      else
      {
        Toast.MakeText(this, "WhatsApp no está instalado. No se pudo enviar el mensaje", ToastLength.Long).Show();
      }
    }
  }
}