Configurer.cs

namespace Configurer
{
    public static class SomeThing
    {
        public static string SomeString { get; set; }
    }
    
    public class PreApplicationStartCode
    {
        public static void Start()
        {
            SomeThing.SomeString = "I was initialized";
        }
    }
}

gistfile1.cs

using System.Web;
[assembly: PreApplicationStartMethod(typeof(PreApplicationStartCode), "Start")]