For the article Writing Custom Control in new WPF XAML Designer I wrote the example app where I needed automatically to restore NuGet package. Write NuGet.Config file in folder where is *.sln file, and write it like:
<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageRestore>
    <add key="enabled" value="True" />
    <add key="automatic" value="True" />
  </packageRestore>
  <packageSources>

    <clear />

    <add key="My Private NuGet Server" value="package" />

  </packageSources>  
</configuration>
From here.