<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="MyKey" value="MyValue"/>
  </appSettings>
  <connectionStrings>
    <add name ="MyHost"
         providerName="System.Data.Sql"
         connectionString="Data Source=(local)SQLEXPRESS;Initial Catalog=Northwind;
                           Integrated Security = true; User ID=Admin;Password=P@ssw0rd;
                           Connection Timeout=2;Encrypt=True;TrustServerCertificate=True" />
  </connectionStrings>
</configuration>

ConnectionStringSettingsCollection settings = ConfigurationManager.ConnectionStrings;
            if(settings != null)
                MessageBox.Show(settings[0].ConnectionString, "ConnectionString");

string s = ConfigurationManager.AppSettings["MyKey"];