The way I would do it, you create a trigger in mission templates file when a player joins the server. Check that he is an admin (player_is_admin) operation. Then broadcast the message (string) to the other players in the server. This would require module system and basic knowledge of it, so best to start there. If you don’t know anything or little about it maybe I or someone else could write out the code for you, but I definitely recommend getting into learning it.