Put this under OnPlayerText callback:
if(strcmp(text,"!pcj",true) == 0)
{
    new Float:x,
          Float:y,
          Float:z,
          Float:angle,
          string[64],
          pveh[MAX_PLAYERS] = 0;
    GetPlayerPos(playerid,x,y,z);
    GetPlayerFacingAngle(playerid,angle);
    pveh[playerid] = CreateVehicle(191,x,y,z,angle,-1,-1);
    PutPlayerInVehicle(playerid,pveh[playerid]);
    SendClientMessage(playerid,0xFFFFFFFF,"Spawned a PCJ-600!");
    return 0;
}
In-game type !pcj.