public Uri HtmlURI {get; set;}

public void ShowCustomPopup()
{
  //Get the active map view.
  var mapView = MapView.Active;
  if (mapView == null)
    return;

  //Create custom popup content
  var popups = new List<PopupContent>();
  popups.Add(new PopupContent("<b>This text is bold.</b>", "Custom tooltip from HTML string"));
  popups.Add(new PopupContent(new Uri("http://www.esri.com/"), "Custom tooltip from Uri"));
  
  mapView.ShowCustomPopup(popups);
}