Dear Laru3ns,
despite being the U21 manager, you can't advertise players who aren't on your actual U21/NT team on the forum. (As a matter of fact, i can't find anywhere in the rules that NT/U21 managers can advertise players on the forum either). I Asked GM-LoD the question last season if advertising was allowed, and he told me that the rules didn't state that U21/NT managers had the right to advertise players. If their would be an exception to this rule, it would have been stated in the Game manual.
However, I've also been told that some GM's do accept advertising NT/U21 players, provided that they are on the U21/NT team! This player isn't on the U21/NT team, and their for, can't be advertised, even tho he's a nice prospect.
What you can do however, is ask if their are any manager who are training a Centre, guard, SF (you name it) and who might be interested in a player. IF they are looking for one, you can ask them to contact you. Upon being contacted, you can mail the players with details. Ofcourse, always be carefull, since they can still report you for spam :P But it's hard to consider something spam if you contacted a U21/NT coach for details about a player and he replied you...
Gm-Revo