//At the top of the Gamemode:
new Pickup;

//Put this next to your other pickups, in OnGameModeInit :)

Pickup = CreatePickup(,2,1743.1935,-1849.4183,13.5812);
                    //^PICKUP MODEL GOES THERE
//Now, put this at the very bottom of your script
public OnPlayerPickUpPickup(playerid, pickupid)
{
    if(pickupid == Pickup)
    {
    //Do whatever here, like send a message, etc :)
    SendClientMessage(playerid,COLOR_YELLOW,"Hey");
    }

}

//Then save & compile :D