@KristyN: That won't work. You are missing quotes:
The easiest is to have WinSCP GUI generate a code template for you:
https://winscp.net/eng/docs/ui_generateurl#script
/command "open sftp://mylogin:mypassword@99.99.99.123"
The easiest is to have WinSCP GUI generate a code template for you:
https://winscp.net/eng/docs/ui_generateurl#script