If I'm not mistaken, generally for bivouacs in Slovenia there are no reservations and it works on the principle of first come, first served.
But you have to be careful with this bivouac at Prehodavci, because the above applies only off-season, when the hut is closed. When the hut is open, I think they consider the bivouac as additional bed capacity besides the hut and then you have to reserve a bed in the bivouac as well. In the online reservation system, you can choose between a bed in the hut itself and a bed in the bivouac.