How to access configuration variables before building the WebApplication builder in .NET 6.0?

Question

So my question is, how would I access the configuration before building the builder in .NET 6?

In .NET 5, we had Startup.cs broken up into three functions:

• public Startup()
• public void ConfigureServices(IServiceCollection services)
• public void Configure(IApplicationBuilder app)

With this setup, I was able to declare a private readonly variable to store and use the configuration throughout Startup.cs

private readonly IConfigurationRoot _config; // The variable in question.
public Startup(IWebHostEnvironment env)
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(env.ContentRootPath)
            .AddJsonFile($"appsettings.json", optional: false, reloadOnChange: true)
            .AddEnvironmentVariables();

        _config = builder.Build(); // Initializing the configuration to be access in ConfigureServices()
    }

I would use that _config variable often while setting up the services... for example:

// Example .NET 5 ConfigureServices.
services.AddSession(options =>
        {
            options.Cookie.Name = _config["Auth:AzureAd:SessionCookieName"];
            options.Cookie.HttpOnly = true;
            options.Cookie.IsEssential = true;
        });

However, the only way I know the access the configuration in Program.cs in .NET 6 is after creating the WebApplication builder var app = builder.Build();

The issue is, you cannot modify services after building the builder. So I have all of these service configurations before this line of code that requires access to appsettings/the configuration file.

So my question is, how would I access the configuration before building the builder in .NET 6?