We always add a key with a value in our XML configuration and read it at run time but we can extend configuration settings in our XML configuration. To do this, we must create a custom configuration section handler. The handler must be a .NET Framework class that inherits from the  System.Configuration.ConfigurationSection class. The section handler interprets and processes the settings that are defined in XML configuration elements in a specific section of a Web.config file.We can read and write these settings through the handler's properties.
 

Here is our handler : 

using System.Configuration;

namespace ConfigurableModule
{
    public class Configuration : ConfigurationSection  
    {
        public static Configuration GetConfiguration()
        {
            Configuration configuration = 
                ConfigurationManager
                .GetSection("myConfigurableModule")
                as Configuration;

            if (configuration != null)
                return configuration;

            return new Configuration();
        }

        [ConfigurationProperty("message", IsRequired = false)]
        public string Message
        {
            get
            {
                return this["message"] as string;
            }
        }
    }
}

Here is our applications configuration file (app.config/web.config).

<configuration>
    <configSections>
        <section name="myConfigurableModule" type="ConfigurableModule.Configuration, ConfigurableModule"/>
    </configSections>
    <myConfigurableModule message="Hello world!" />
    <!-- Optionally other configuration -->
</configuration>

We can get the Message property from the configuration like this:

string messageFromConfiguration = ConfigurableModule.Configuration.GetConfiguration().Message ;

Happy coding :)