It depends on which program number you use.

If you use MP, then it will automatic show you are a *G. Then you can request (but it is up to the reservation agent if he or she will confirm that for you).

Standard policy is 24 hours except OP Elites. But if you ask nicely, usually they will give it (bulkhead, not exit row) confirmed for free.

But if you book by OP and hold no elite status, good luck.