Here is a script you can call, su plex -s bin/sh -c /path/to/script.sh
Amend as appropriate
#!/bin/sh
export LD_LIBRARY_PATH="/var/packages/Plex Media Server/target"
export PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR="/volume1/Plex/Library/Application Support"
"/var/packages/Plex Media Server/Plex Media Scanner" ---your command options or `$1-` here---