To design an external login feature in .NET, you will need to:

1. Register your application with the external login provider. This will give you a client ID and client secret that you will need to use to authenticate users with the provider.
2. Configure your application to use the external login provider. This involves adding the client ID and client secret to your application's configuration file.
3. Create a login button or link that users can click to sign in with the external login provider.
4. Handle the login event. This event will be raised when a user successfully logs in with the external login provider.

Here are some additional details about each step:

1. To register your application with an external login provider, you will need to visit the provider's website and create an application. Once you have created an application, you will be given a client ID and client secret.
2. To configure your application to use an external login provider, you will need to add the client ID and client secret to your application's configuration file. The configuration file is typically located in the appsettings.json file.
3. To create a login button or link that users can click to sign in with the external login provider, you can use the following code:

Code snippet

<a href="/Account/ExternalLogin">Sign in with external provider</a>

1. To handle the login event, you will need to subscribe to the ExternalLogin event in your application's Startup class. The following code shows how to do this:

Code snippet

protected void ConfigureServices(IServiceCollection services)
{
    services.AddAuthentication(options =>
    {
        options.DefaultScheme = "External";
        options.DefaultSignInScheme = "External";
    });

    services.AddMvc();
}

protected void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseAuthentication();

    app.UseMvc();

    app.Run(async (context) =>
    {
        await context.SignInAsync("External");

        // Redirect the user to the home page.
        context.RedirectToAction("Index");
    });
}

Once you have completed these steps, your application will be able to allow users to sign in with external login providers.