[Jstylezzz]

pawn Код:

CMD:setplate(playerid,params[])
{
    new string[128],plate[128];
    if(sscanf(params, "s", plate)) SendClientMessage(playerid, 0xF97804FF, "USAGE: /setplate [plate]");
    if(!IsPlayerInAnyVehicle(playerid)) SendClientMessage(playerid,COLOR_RED,"You need to be in a vehicle in order to use this..");
    SetVehicleNumberPlate(vehicleid, plate);
    format(string,sizeof(string),"Numberplate setted to: %s",plate);
    SendClientMessage(playerid,COLOR_GREEN,string);
    return 1;
}

Not tested but should work..
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