Справочник по конфигурациям в C# — 9.1. Основные секции

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <config>
    <add key="repositoryPath" value="./packages" />
    <add key="http_proxy" value="http://proxy.corp.com:8080" />
    <add key="no_proxy" value="localhost,127.0.0.1" />
  </config>

  <packageSources>
    <clear />
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
    <add key="internal" value="https://nuget.corp.com/v3/index.json" />
  </packageSources>

  <packageSourceCredentials>
    <internal>
      <add key="Username" value="user" />
      <add key="ClearTextPassword" value="pass" />
    </internal>
  </packageSourceCredentials>

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

  <disabledPackageSources>
    <add key="legacy" value="true" />
  </disabledPackageSources>
</configuration>