If the program name contains spaces or other non alphanumeric characters it is shortened to the DOS compatibility Name.
@="C:\\Program Files (x86)\\PathToClipboard.exe %1"
The quotes should be around the individual parts, not the complete string.
In this particular case, you should only quote the first part (program name) and the whole string:
@="\"C:\\Program Files (x86)\\PathToClipboard.exe\" %1"
Note:
\
is used to escape the inner quotes (those around the program name)- If you also quote the
%1
(the filepath) it will add the quotes when the filepath is passed to the program, so don't quote this part.