First make a besiege creation with random blocks where you want the hidden blocks to go (recommend placing it last because it will be the last in the file later) then go to your steam library, right click besiege, go to properties, local files, browse local files, besiege_data, saved machines, open the besiege creation with wordpad or some other document program, find prefab ids, find the block your looking for (the last one if you did as i recommended), replace it with one of the hidden blocks id, save the file and re-load your creation and then your done. (If you want to use the rope, then as a random block place a brace where you want the rope to be).

how do you get the hidden blocks