How to add an assembly reference?

Hi,

I have a Xamarin.Forms PCL project, when I use "System.ServiceModel.MessageHeaderAttribute", I got an error for it, the error message is
"Error 125 The type or namespace name 'MessageHeaderAttribute' does not exist in the namespace 'System.ServiceModel' (are you missing an assembly reference?)"
But it is OK in an asp.net MVC project.
so how can I add this reference in Xamarin.Forms PCL project?

Thanks

Ting