//Startup.cs
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
      .....
      app.UseStaticFiles(new StaticFileOptions
      {
          FileProvider = new PhysicalFileProvider(
               Path.Combine(Directory.GetCurrentDirectory(), "static")),
               RequestPath = "/static"
      });
      .....
}

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/static-files?view=aspnetcore-5.0

ถ้ามีการ authen ก่อน

เอาโค้ดไว้ล่าง app.UseAuthentication();

https://dev.to/j_sakamoto/how-can-i-protect-static-files-with-authorization-on-asp-net-core-4l0o