Web applications using HTTP/HTTPS, like ASP.NET, are always request/response type apps. The client makes a request and the server sends a response to it. A web server NEVER arbitrarily sends anything to a client without the client making a request for it.
So, in a direct sense, no, you can't send a message from the web app to the client.
But, you can using other technologies, like
SignalR[
^].