You can specify a password as part of the session URL:
https://winscp.net/eng/docs/session_url

Anyway it is not safe at all. Afaik, the command line arguments used to run a process is a public information available to all other processes on the machine.

If you had a parameter for the password in the command line then I can maintain and secure my passwords as I see fit. And my script program will be the only process with access to the password.