Just tried it and seems to work normally.
You should be able to visit your crafting station and on the end you'll see a key symbol. This opens the menu to craft it. It takes one Essence summoner plug, the broken key, Ecto essence, and bones. The key when fixed is called "Essence Gate Key"., not to be confused with the "Essence Cavern Key". If the option to craft it does not show, this is an known issue with old saves. Let me know if that's the case, feel free to send me your save file to the email you got the Special Code from, and I'll be more than happy to add it manually.