iOS NSAttributedString rounded background with color

Question

I would like to create something similar as seen on this image:

I managed to create evertyhing with NSMutableAttributedString, except the orange rounded rectangle. I can set the background to that color with BackgroundColor, but I can't find a way to make it rounded. Any samples how can I achieve this? I found this answer, but I can't make it work in Monotouch: NSAttributedString background color and rounded corners

My code in Xamarin.iOS:

var stringBuilder = new NSMutableAttributedString();
stringBuilder.Append(new NSAttributedString("26"));
stringBuilder.SetAttributes(new UIStringAttributes(){BackgroundColor = UIColor.Orange}, new NSRange(stringBuilder.Length - appendString.Length, appendString.Length));
label.AttributedText = stringBuilder;