One example of custom section in configuration file.

App.Config:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  
  <configSections>     
    <section name="links" type="System.Configuration.NameValueSectionHandler">
    </section>
  </configSections> 
  
  <links>
      <add key="link1" value="http://milosev.com/25-c.html" />
      <add key="link2" value="http://milosev.com/84-asp-net-mvc-3.html" />
  </links>
  
  <appSettings>
      <add key="homePage" value="http://milosev.com/" />
  </appSettings>    
  
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
    </startup>
</configuration>
.NET code:
using System;
using System.Collections.Specialized;
using System.Configuration;

namespace CustomConfig
{
    class Program
    {
        static void Main(string[] args)
        {
            NameValueCollection links = ConfigurationManager.GetSection("links") as NameValueCollection;
            string homePage = ConfigurationManager.AppSettings.Get("homePage");

            Console.WriteLine("Home page: " + homePage);

            foreach (string link in links)
            {
                Console.WriteLine("Link: " + links.Get(link));
            }

            Console.WriteLine("Press any key...");
            Console.ReadKey();
        }
    }
}
In reference list you will need to add System.Configuration.

POI:

  <configSections>     
    <section name="links" type="System.Configuration.NameValueSectionHandler">
    </section>
  </configSections> 

Here is more about configSection.

Also notice line:

Console.WriteLine("Link: " + links.Get(link));