In need a Message for user after Upload

I have a problem with the acceptance of the nextcloud by many users. Many do not understand that they need to share a file after the upload. I have an external help page involved, but nobody likes that. Therefore, I am looking for a solution to display a screen message (such as a popup) after the upload to alert the user that he must share now.
I know that’s actually ridiculous, but the users do not want to accept that. Does anyone have an idea how I can solve this?