NAV MESSAGE USER WHO IS LOGGED IN

The build-in way is through notifications. You can insert a notification into the Record Link table and it will be shown in "My Notifications". But it will not be a message popping out, it will just show up in user's "My Notification" factbox - if the factbox is included and open on user's Role Center,

Check the codeuint 454 to see how to code it.

If you want an actual message to show up as if generated by MESSAGE command you need to add a code looking every so often into some table for the new messages, and once something appears in it show it using MESSAGE and mark it as shown in the table. The code needs to be "self-run" to make it work, you would probably need to use PingPong add-on on your Role Centre page to facilitate this.

For an example of how to use PingPong add-on look into page 681. Just bear in mind that this sort of notifications via a pop-up message appearing out of nowhere is very annoying for the users.