Get ConnectionString from appsettings.json instead of being hardcoded in .NET Core 2.0 App

Question

I have the following class in NET Core2.0 App.

// required when local database does not exist or was deleted
public class ToDoContextFactory : IDesignTimeDbContextFactory<AppContext>
{
    public AppContext CreateDbContext(string[] args)
    {
        var builder = new DbContextOptionsBuilder<AppContext>();
        builder.UseSqlServer("Server=localhost;Database=DbName;Trusted_Connection=True;MultipleActiveResultSets=true");
        return new AppContext(builder.Options);
    }
}

This is required in Core 2.0 with migration when Database does not exist and has to be created when you run update-database.
Unable to create migrations after upgrading to ASP.NET Core 2.0

I would like not having ConnectionString in 2 places(here and in appsettings.json) but only in .json so I have tried to replace

"Server=localhost;Database=DbName;Trusted_Connection=True;MultipleActiveResultSets=true"

with

ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString

but it's not working, I'm getting null value.

UPDATE 1:
Just to note that adding explicitly .json is not necessery in Core 2 so the problem is not with the file.
https://andrewlock.net/exploring-program-and-startup-in-asp-net-core-2-preview1-2/

UPDATE 2:
Also I am already using Configuration for sending ConnectionString from .json to Context:

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

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<AppContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
    }
}

But I can not use this for ToDoContextFactory because it does not have Configuration, and ToDoContextFactory is used by migrations so the App is not running at all.

SOLUTION: Based on answer from @JRB I made it work like this:

public AppContext CreateDbContext(string[] args)
{
    string projectPath = AppDomain.CurrentDomain.BaseDirectory.Split(new String[] { @"bin\" }, StringSplitOptions.None)[0];
    IConfigurationRoot configuration = new ConfigurationBuilder()
        .SetBasePath(projectPath)
        .AddJsonFile("appsettings.json")
        .Build();
    string connectionString = configuration.GetConnectionString("DefaultConnection");

    var builder = new DbContextOptionsBuilder<AppContext>();
    builder.UseSqlServer(connectionString);

    return new AppContext(builder.Options);
}

Answer 1

STEP 1: Include the following in OnConfiguring()

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        IConfigurationRoot configuration = new ConfigurationBuilder()
            .SetBasePath(AppDomain.CurrentDomain.BaseDirectory)
            .AddJsonFile("appsettings.json")
            .Build();
        optionsBuilder.UseSqlServer(configuration.GetConnectionString("DefaultConnection"));
    }

STEP 2: Create appsettings.json:

  {
    "ConnectionStrings": {       
      "DefaultConnection": "Server=YOURSERVERNAME; Database=YOURDATABASENAME; Trusted_Connection=True; MultipleActiveResultSets=true"        
    } 
  }

STEP 3: Hard copy appsettings.json to the correct directory

  Hard copy appsettings.json.config to the directory specified in the AppDomain.CurrentDomain.BaseDirectory directory. 
  Use your debugger to find out which directory that is.        

Assumption: you have already included package Microsoft.Extensions.Configuration.Json (get it from Nuget) in your project.