If you're set on making it a post-game bonus dungeon, I would recommend making the players aware of it during the cinematic involving the final boss. Perhaps you could have an area in the game like a mountain pass with a blocked off cave, or a shrine with a sealed door. Just make sure its in a place that the player will have seen throughout their journey through your game, and will be recognizable. Then when you encounter the boss, something happens, like maybe a giant storm surrounding the world resulting in a lightning bolt striking the blocked off cave, causing the barrier to burn off and allowing access. Or you defeat the boss, and it turns out that long ago, they were the guardian of the sealed shrine, and in defeating them, the seal has broken. Then credits roll and the final story based cinematics play, but the game itself doesn't end, and the player is able to return to that place where the dungeon is located and enter it.
However, I feel like bonus dungeons are better suited as a "during the game" kind of experience. Still make it something that's easily visible, but make it something that requires a quest to open. Something like "Blah blah blah great evil sealed behind this door only I can break the seal - *Insert name of old man you encountered early on in the game*". This way the player will have to remember some random (typically unmemorable) NPC, and run back to them in order to get it opened. Don't make it super easy though, have the old man give them a test to prove they can handle themselves in the dungeon, or have them need to find the key to open the chest containing the item that will break the seal because he lost it or something, get creative with it. And then throw in an additional thing where clearing the final dungeon makes the final boss even stronger.