How to read appsettings.json value in ConfigureAppConfiguration (Azure function startup.cs)

I have an Azure Function App that reads the value of appsettings.json and replaces part of the secret with Azure Key Vault

However, the URI and userAssignedClientId used by KeyVault need to get the value from the appsetting.json file according to the environment

How should I change the URI and userAssignedClientId here to come from appsettings.json

This is my Startup.cs

using Azure.Extensions.AspNetCore.Configuration.Secrets;
using Azure.Identity;
using Microsoft.Azure.Functions.Extensions.DependencyInjection;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using System;
using System.IO;

[assembly: FunctionsStartup(typeof(FunctionApp3.Startup))]
namespace FunctionApp3
{
    public class Startup : FunctionsStartup
    {
        public override void Configure(IFunctionsHostBuilder builder)
        {
            builder.Services.AddOptions<Settings>()
                .Configure<IConfiguration>((settings, configuration) =>
                {
                    configuration.GetSection("Settings").Bind(settings);
                });
        }

        public override void ConfigureAppConfiguration(IFunctionsConfigurationBuilder builder)
        {
            FunctionsHostBuilderContext context = builder.GetContext();

            string userAssignedClientId = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"; //Here
            var credential = new DefaultAzureCredential(new DefaultAzureCredentialOptions { ManagedIdentityClientId = userAssignedClientId });

            var options = new AzureKeyVaultConfigurationOptions { ReloadInterval = TimeSpan.FromHours(24) };

            builder.ConfigurationBuilder
                .AddJsonFile(Path.Combine(context.ApplicationRootPath, "appsettings.json"), optional: true, reloadOnChange: false)
                .AddJsonFile(Path.Combine(context.ApplicationRootPath, $"appsettings.{context.EnvironmentName}.json"), optional: true, reloadOnChange: false)
                .AddEnvironmentVariables()
                .AddAzureKeyVault(new Uri("https://xxxxxxxx.vault.azure.net/"), credential, options); //Here

        }
    }
}

Thanks to both of you for the inspiration, I later solved it with the following code

using Azure.Extensions.AspNetCore.Configuration.Secrets;
using Azure.Identity;
using Microsoft.Azure.Functions.Extensions.DependencyInjection;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using System;
using System.IO;

[assembly: FunctionsStartup(typeof(ADX_Function.Startup))]
namespace ADX_Function
{
    public class Startup : FunctionsStartup
    {
        public override void Configure(IFunctionsHostBuilder builder)
        {
            builder.Services.AddOptions<Settings>()
                .Configure<IConfiguration>((settings, configuration) =>
                {
                    configuration.GetSection("Settings").Bind(settings);
                });
        }

        public override void ConfigureAppConfiguration(IFunctionsConfigurationBuilder builder)
        {
            FunctionsHostBuilderContext context = builder.GetContext();

            var configurationBuilder = builder.ConfigurationBuilder
                .AddJsonFile(Path.Combine(context.ApplicationRootPath, "appsettings.json"), optional: true, reloadOnChange: false)
                .AddJsonFile(Path.Combine(context.ApplicationRootPath, $"appsettings.{context.EnvironmentName}.json"), optional: true, reloadOnChange: false)
                .AddEnvironmentVariables()
                .Build();

            string userAssignedClientId = configurationBuilder["userAssignedClientId"];
            var credential = new DefaultAzureCredential(new DefaultAzureCredentialOptions { ManagedIdentityClientId = userAssignedClientId });

            var options = new AzureKeyVaultConfigurationOptions { ReloadInterval = TimeSpan.FromHours(24) };

            builder.ConfigurationBuilder
                .AddJsonFile(Path.Combine(context.ApplicationRootPath, "appsettings.json"), optional: true, reloadOnChange: false)
                .AddJsonFile(Path.Combine(context.ApplicationRootPath, $"appsettings.{context.EnvironmentName}.json"), optional: true, reloadOnChange: false)
                .AddEnvironmentVariables()
                .AddAzureKeyVault(new Uri(configurationBuilder["KeyVaultUri"]), credential, options);

        }
    }
}