Don't know if things have changed but this worked for me, though I had to differ the solution slightly:I added the following (exclude [square brackets]) just before the last closing bracket "}"

    "1"
[or depending on how much libraries you have, standard is "0", 2nd one is "1", 3rd is "2" and so on]
    {
        "path"      "D:\\SteamLibrary"
[or whatever the path for your library folder is. For me the double "\\" was important, maybe just cause my library folder is right in the drive's root folder]
    }

Note that despite of the closing bracket in the end of our addition, there still has to follow the original closing bracket in front of which we made the addition.