[Lordzy]

There are several other methods to do what you're trying to. I'm up late and this is what I've got in my quick drowsy thinking -

pawn Код:

CMD:whisper(playerid, params[]) {

    new
        target[MAX_PLAYER_NAME], //You're gonna take in ID/name as a string.
        text[128],
        ID //To either be used as player ID or mask ID.
    ;
    
    if(sscanf(params, "s["#MAX_PLAYER_NAME"]s[128]", target, text))
        return SendClientMessage(playerid, COLOR_GREY, "USAGE : /whisper [playerid/name/maskid] [text]");
        
    if(!IsNumeric(target)) //If the second argument given is not numeric, it's probably a player name?
        ID = GetPlayerNameByID(target); //Build a custom function that gets player ID from player name.
    else
        ID = strval(ID); //If it's an integer, it could either be player ID or mask ID.
        
    if(ID > MAX_PLAYERS) //Like you mentioned earlier, if it's greater than MAX_PLAYERS..
        //It's a mask ID.
    else
        //It's a player ID.
        
    return 1;
}

Another idea to do it within sscanf itself is to create a custom sscanf specifier.