I don't know of any way to get a lost key that was never registered in our lobby back other than purchasing the game again sadly, afaik on Steam you'll have the option to copy/see the product/online key when right clicking the game in your library, but I can't check that myself.
At least the good news is that having already downloaded all of the ABB content you'll have the archive folder with all the compressed files and would merely need to change the GTL Install directory within the Autoupdater and select Reinstall/Neuinstallation for it to simply install the mods without needing to slowly download them again, and installing alone won't take too long.
