How to get TFS2015 Build (Build.vnext) and NuGet package restore to use custom package sources
Alternatively you could also add a NuGet Installer build step before the Visual Studio Build step in your build configuration to restore all NuGet packages. There you can pass the location of your private repo as argument to nuget.exe: -source “https://www.nuget.org/api/v2/;http://mynugetserver”