How to create a pop up message box in nopCommerce plug

I am new for nopCommerce plug.

I want to create a pop up message box during action in my plug in.
I created cshtml under view, model, and in controller I created partialView, but nothing popup

Please read all of this closely
https://docs.nopcommerce.com/en/developer/plugins/index.html

And then check out this:
https://www.w3schools.com/jsref/met_win_alert.asp

chen.lu wrote:
Please clarify "during action".

Are you aware of the NotificationService?  It will put your message in the Notification bar (top of page), but it will hide after some timeout.
There are several Examples in
\Plugins\Nop.Plugin.Misc.Zettle\Controllers\ZettleAdminController.cs
_notificationService.WarningNotification(warning, false);

If you need a custom pop up, then be aware that an action method (in controller) will just set some properties of the model or can set the ViewData or ViewBag or TempData (the latter is what the NotificationService uses).  It's the View (.cshtml) file that will need to look for those and then use JavaScript to pop up the message.