[MJ!]

At OnPlayerRequestClass

pawn Code:

if(classid == YOURSKIN)
{
   ShowPlayerDialog(playerid,1,DIALOG_STYLE_INPUT,"Skin","This skin requers a password !\nType down the correct password to choose it !,"OK","Cancel");
}

At OnDialogResponse

pawn Code:

if(response)
{
  switch(dialogid)
  {
     case 1:
     {
       if(strval(inputtext) == 99999) 
       {
           SendClientMessage(playerid,COLOR_WHITE,"Correct Password. Skin taken.");
           SetPlayerSkin(playerid,YOURSKIN);
           TogglePlayerControllable(playerid,true);
           TogglePlayerSpectating(playerid,0);
           SetCameraBehindPlayer(playerid);
           SpawnPlayer(playerid);
        }
        else
        {
           SendClientMessage(playerid,COLOR_GREY,"Invalid Password !");
           return 1;
        } 
     }
   }
}

99999 it's the password correct