To add the System.Uri right-click on the Constructor Node, then navigate to Select .NET Framework Constructor in the opened window.
Click on Browse, and navigate to this path: C:\Windows\Microsoft.NET\Framework64\v4.0.30319\System.dll, then from the Object double-click on System and choose .Uri.
You will see the list of Constructors, select the one which is more suitable for your project (f.e, Uri(String uriString)), after completing the steps, press OK.
