In .NET Core, you can retrieve values from the appsettings.json file using the Configuration object provided by the Microsoft.Extensions.Configuration package. Here's how you can do it:

1. Add a reference to the Microsoft.Extensions.Configuration package in your project.
2. In your Startup.cs file, within the ConfigureServices method, add the following code to configure the IConfiguration object:

csharpCopy codeusing Microsoft.Extensions.Configuration;

public class Startup
{
    private IConfiguration Configuration { get; }

    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public void ConfigureServices(IServiceCollection services)
    {
        // Configure the IConfiguration instance
        services.AddSingleton(Configuration);
        
        // Other service configurations...
    }

    // Other methods...
}

1. In your code, you can now access the values from the appsettings.json file by injecting the IConfiguration object into your classes or methods. For example:

csharpCopy codeusing Microsoft.Extensions.Configuration;

public class MyClass
{
    private IConfiguration _configuration;

    public MyClass(IConfiguration configuration)
    {
        _configuration = configuration;
    }

    public void MyMethod()
    {
        // Access a value from the appsettings.json file
        var myValue = _configuration["MyKey"];
        
        // Other code...
    }
}

In the above code, MyKey represents the key of the value you want to retrieve from the appsettings.json file. Make sure the appsettings.json file is correctly configured with the key-value pairs you need.

Remember to build and run your application, ensuring that the appsettings.json file is located in the project's root directory or a subdirectory.