Pop up to report private messages

On the Proximus forum, we are often asked to reply to private messages. 
At our last meeting, one of our colleagues suggested this idea: 
- When we log in at the beginning of the day on the forum frontend or via the insided interface, would it be possible to have a pop-up to warn us that we have new private messages? 
At the moment, we have a red bubble with the number of private messages but it would be interesting to have this little pop-up...