You can buy your ticket from DB, and buy it without reservation, and then buy the reservation separately from ÖBB.

But I have found railway lounges always rather underwhelming (understandably as waiting non trivial amounts of time for a train is not very common). I would just head for a proper bar/restaurant.

Also, if you are travelling from somewhere else to Munich by train as well, than maybe you should book from that somewhere else to Jenbach.