Startup.cs

public void Configure(IApplicationBuilder app)
        {
            app.UseSpa(builder =>
            {
                builder.Options.SourcePath = "frontend";
            
                if (Environment.IsDevelopment())
                {
                    builder.UseProxyToSpaDevelopmentServer("http://localhost:3001");
                    // builder.UseReactDevelopmentServer("start");
                }
            });
}