You are possibly using old version of WinSCP that does not support -username.
If you have the latest version and it still does not work, please post your code.

Thank you Martin!

Does the -username work with the open sftp://<Site> command in some way?

I'm getting error:

Unknown switch 'username'.

(But substituting %%40" for @ seems to work).

- Joe P.

The literal @ in session URL should be URL-encoded in general:
https://winscp.net/eng/docs/session_url#special
(Though in this specific case, WinSCP should handle an unencoded @ too)

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Or use -username switch:
https://winscp.net/eng/docs/scriptcommand_open#username

Hello Martin, WinSCP Support,

Have a new file transfer to setup, and I use the automation script file method, building the
connection string on the fly, automated.

This new file transfer, has the character @ in the username, so this statement will say something like

Do I need to add logic to escape the first @ character? Is that needed, or what is the recommendation?

Thanks,
Joe P.