Using ConfigurationManager

file.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Configuration;

namespace browser
{
    public partial class Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            var value = ConfigurationManager.AppSettings["PrimaryConnection"];
        }
    }
}

web.conf

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <startup> 
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
  </startup>
  <appSettings>
    <add key ="PrimaryConnection" value ="SomeData" />
  </appSettings>
</configuration>