Agree the script is failing because of the curly-bracket escapes that are needed, but I also think this will probably not do what you expect.
HKEY_USERS\.Default hive is not the Default Profile used when new users log on. It’s the profile of the LocalSystem account. See https://devblogs.microsoft.com/oldnewthing/20070302-00/?p=27783 for details.
To make a change that applies to all User Profiles, I’d recommend using Local Group Policy (example https://bigfix.me/fixlet/details/26933), or Startup/Logon Script, or ActiveSetup (example Action That Runs Once per User ) , or (worst-case) use ‘reg load’ to load the ntuser.dat to a new Hive, update that Hive, and then use ‘reg unload’ to unload the ntuser.dat for the Default User profile.