Спасибо, LifeStyle.И окончательная реализация для Xamain:
[iOS]
public void Send(string message)
{
var activityItems = new[] { NSObject.FromObject(message) };
var activity = new UIActivityViewController(activityItems, null);
UIApplication.SharedApplication.KeyWindow.RootViewController.PresentViewController(activity, true, null);
}
[Android]
public void Send(string message)
{
var intent = new Intent(Intent.ActionSend);
intent.SetType("text/plain");
intent.PutExtra(Intent.ExtraSubject, "Subj");
intent.PutExtra(Intent.ExtraText, message);
_targetActivity.StartActivityForResult(Intent.CreateChooser(intent, "Title"), _resultCode);
}