This is the source of the Workshop uploader contained in all versions of After the Collapse before 0.7. While it probably isn't too much to work with, we're releasing this in the event that it helps someone with their game.
You can use TakeThree as part of your game under the terms of the Microsoft Reciprocal Source License (included with this source distribution). You'll probably want to change around the UI a bit though, as it's pretty much unchanged from its use in After the Collapse.
You'll need to restore NuGet pacakges for your solution, change the AppID in steam_appid.txt, and extract steam_api64.dll from here (the file you're looking for is in Release\net46) and drop it in your bin/ folder.
If you have any further questions, feel free to ask us - we're usually hanging out in our Discord server or you can file an issue.
Have fun!