Any of Parsec's advanced settings can be passed on the command line instead of being set in the app. See All Advanced Configuration Options for the full list of available settings.

Separate multiple settings with colons. For example:

"C:\Program Files\Parsec\parsecd.exe" peer_id=exampleid:encoder_bitrate=30

Replace exampleid with the peer ID of the computer you want to connect to, available in the Computers tab. The executable path differs by platform. For the correct path on macOS and Linux, and for how to launch Parsec from the command line, see Run Parsec App From Command Line.