What’s the standard way to show downloading notification to user for file downloads that require authorization in web applications?

Our web application needs to enable secured file download by requiring HTTP requests to have authorization token.

Due to this, when user clicks download the file will be retrieved using AJAX GET request with the access token. Once the file is in memory, it will be saved to user's hard drive using temporary link pointing to file's data in memory.

Because of this approach, browser doesn't show download progress immediately like in normal file download (in the bottom bar in Chrome). We need a way to tell user that something is going on as it may take some time to retrieve the file data.

We are thinking of showing a toast message and dismiss it when done. I haven't seen many examples of this on the net in many websites.

What is the standard way (if there is any) to do this?