I don't think calibre-portable.exe supports command line arguments.
Two options.
- You can create a symlink of "My Main Calibre Library" inside the "Calibre Portable" directory. Calibre Portable knows how to "portabilize" those
- Do the entire "portabilization" in a batch file. You can use "\Calibre Portable\Calibre\resources\calibre-portable.bat" for reference.