See https://winscp.net/eng/docs/scripting#hostkey

Ordinarily WinSCP will prompt us whether we trust and save the security key data of the server when we login the server the first time. This is not a problem for normal console mode usage, while if we need to operate some new hosts with batch scripts, it's a big trouble, because WinSCP will still prompt whether to trust those untrusted servers even in batch mode, and we have no option to let WinSCP choose to trust the host, it can only abort the job. Is it possible that we can explicitly tell WinSCP trust the untrusted hosts and save their security key automatically? Currently we must login every new host manually and tell WinSCP to trust them, before we can operate with them using batch scripts. This is extremely annoying for automation works.
Thanks!