Hi Khaled
The intended way of operation by design is for the intro prompt to start playing with the MOH simultaneously.
The MOH volume is lower at that moment, and when the intro is over the volume goes back to normal, so that they are mixed together for a nicer effect.
If you want to play a separate message, you can send the caller to an IVR, call it say "Intro Message" and then make the IVR send the call to your queue.
Another option would be to use a playlist as MOH. In the playlist, the first music file can be silence for a few seconds (to cover for the intro prompt), and the second file can be a music file that is a bit longer than the Maximum Queue Wait Time. The customer will never hear it loop in this case