Add reference System.Net.http

Hi,

I add the reference System.Net.http in my plug-in Eb for develop a communication with a REST API WindChill.

When I want deploy my plug-in I receive this error :

>>>>>> Deployment to Engineering Base <<<<<<
Contacting Engineering Base 6.7.1...
Connection established
Deployment to Engineering Base is running...
Assembly with name System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a hasn't been found. The assembly has to exist in the same directory as the plug-in assembly.

I try to copy the dll file on my directory project and add this dll on reference. Without successes.