Snippet from WebHostBuilderExtensions to show how UseStartup registers the assembly that hosts the Startup class as the application name

WebHostBuilderExtensions.cs

/// <summary>
/// Specify the startup type to be used by the web host.
/// </summary>
/// <param name="hostBuilder">The <see cref="IWebHostBuilder"/> to configure.</param>
/// <param name="startupType">The <see cref="Type"/> to be used.</param>
/// <returns>The <see cref="IWebHostBuilder"/>.</returns>
public static IWebHostBuilder UseStartup(this IWebHostBuilder hostBuilder, Type startupType)
{
    var startupAssemblyName = startupType.GetTypeInfo().Assembly.GetName().Name;

    return hostBuilder
        .UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName)
        .ConfigureServices(services =>
        {
            if (typeof(IStartup).GetTypeInfo().IsAssignableFrom(startupType.GetTypeInfo()))
            {
                services.AddSingleton(typeof(IStartup), startupType);
            }
            else
            {
                services.AddSingleton(typeof(IStartup), sp =>
                {
                    var hostingEnvironment = sp.GetRequiredService<IHostingEnvironment>();
                    return new ConventionBasedStartup(StartupLoader.LoadMethods(sp, startupType, hostingEnvironment.EnvironmentName));
                });
            }
        });
}