To add a parm to OpenLP Portable you need to edit the file:
...\App\Appinfo\Launcher\OpenLPPortable.ini
Where "..." is the Drive\Directory of the portable installation.
Edit the line:
CommandLineArguments="-p"
To produce a debug log, the line would be:
CommandLineArguments="-p -l debug"
The logfile is produced in the directory:
...\Other\openlp.log
ElderP