How can add package in Core of Project Solution in Xamarin

Hi , I am new in Xamarin. Actually I tried too much find in google but not getting appropriate answer. I want to add package in Core of Project Solution in Xamrin Studio for Mac.

 

Actually I want to use "using System.Net.Http; " . But Getting error like that "this assembly is missing".

 

So kindly give the appropriate answer as soon as. 

 

Thanks