how to lock screen until pdf is generated in c#

Question

hi I have this program that has a with a link that, once clicked, starts the download of a file. i need to add a loading screen until the file is downloaded.

i've seen this text solution but when implementing the

Response.Cookies.Add(
    new HttpCookie("downloadStarted", "1") 
    { 
        Expires = DateTime.Now.AddSeconds(20) 
    });

the program stops working (no more fader). also this solution has deprecated lines of code. please help thanks

the button

<a href="@Html.Raw(Url.Action("CarrelloDownload", "Carrello", new {}))", class="btn btn-primary">Scarica</a>