Restore nuget Packages in VSTS

1. Create a nuget.config file(It contains the package source : nuget.org and cutsom package source) as part of the solution

<?xml version="1.0" encoding="utf-8"?>

<configuration>

  <packageSources>

    <!-- remove any machine-wide sources with <clear/> -->

    <clear />

    <!-- add a Team Services feed -->

    <add key="Key" value="TFSUrl/_packaging/Application/nuget/v3/index.json" />

    <!-- also get packages from the NuGet Gallery -->

    <add key="nuget.org" value="https://www.nuget.org/api/v2/" />

  </packageSources>

  <activePackageSource>

    <add key="All" value="(Aggregate source)" />

  </activePackageSource>

</configuration>

2 Add the path of the nuget.config to the nuget restore task in VSTS as shown below