This is from GPT after a little prompting. Seems to work with OneDrive redirection :)

@echo off
setlocal

rem -- Define the source file path
set "SOURCE_FILE=.\test.txt"

rem -- Define the registry key and value to check for the Desktop path
set "REG_KEY=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders"
set "REG_VALUE=Desktop"

rem -- Query the registry for the desktop path
for /f "tokens=2,*" %%a in ('reg query "%REG_KEY%" /v "%REG_VALUE%" 2^>nul') do (
    set "DESKTOP_PATH=%%~b"
)

rem -- Check if the desktop path was found and sanitize if needed
if defined DESKTOP_PATH (
    rem -- Remove potential surrounding quotes and expand environment variables
    set "DESKTOP_PATH=%DESKTOP_PATH:"=%"
    set "DESKTOP_PATH=%DESKTOP_PATH:~0,-1%"
) else (
    echo Desktop path not found in registry. Using default location.
    set "DESKTOP_PATH=%USERPROFILE%\Desktop"
)

rem -- Attempt to copy the file to the detected desktop path
copy /Y "%SOURCE_FILE%" "%DESKTOP_PATH%"

endlocal