Here is the official documentation:
Here is an example (assuming the filename doesn’t change):
PostCustomCommandPath=C:\Windows\System32\cmd.exe
PostCustomCommandParameters="/c copy "%originalfilepath%\%originalfilename%-thumb.jpg" "%destinationpath%\""
PostCustomCommandHangPeriod=60
PostCustomCommandCritical=false
PostCustomCommandUISession=false
PostCustomCommandShowWindow=false
PostCustomCommandExitCodeCheck=false
If you’re on the latest 2.4.9 version you can use "%destinationpath%\%convertedfilename%-thumb.jpg"
if your converted filename is different from the original filename.