OptionsBuilderExtensions.cs

using Microsoft.Extensions.Options;
using System;
using System.Configuration;
using System.Linq;
using System.Reflection;

namespace ConsoleApp
{
    public static class OptionsBuilderExtensions
    {
        public static OptionsBuilder<TOptions> ConfigureFromAppSettings<TOptions>(this OptionsBuilder<TOptions> builder, bool useOptionNameAsPrefix = true) where TOptions : class
        {
            Type optionsType = builder.GetType().GenericTypeArguments.First();

            string prefix = useOptionNameAsPrefix ? $"{optionsType.Name}." : string.Empty;

            builder.Configure(configure =>
            {
                foreach (var property in optionsType.GetProperties(BindingFlags.Instance | BindingFlags.Public))
                {
                    string fullName = $"{prefix}{property.Name}";

                    string appSettingsValue = ConfigurationManager.AppSettings.Get(fullName);
                    property.SetValue(configure, Convert.ChangeType(appSettingsValue, property.PropertyType));
                }
            });

            return builder;
        }
    }
}