Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.DependencyInjection;

namespace HelloSignalR
{
    public class Startup
    {
        readonly string AllowAllPolicy = "AllowAllPolicy";

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddCors(options =>
            {
                options.AddPolicy(AllowAllPolicy,
                    builder =>
                    {
                        builder
                            .AllowAnyMethod()
                            .AllowAnyHeader()
                            .AllowAnyOrigin()
                            .AllowCredentials();
                    });
            });
            services.AddSignalR();
        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            app.UseStaticFiles();
            app.UseCors(AllowAllPolicy);
            app.UseSignalR(routes =>
            {
                routes.MapHub<ChatHub>("/chat");
            });
        }
    }
}