What is "string for the automation"
? Do you mean session URL
The host key is not part of your session settings. It's cached on the first connection. So if you did not connect yet on the other machine, WinSCP does not know the host key and cannot include it in the generated URL/script/code/whatever.
As documented: https://winscp.net/eng/docs/ui_generateurl
WinSCP 5.7.* is over 7 years old. So no wonder it is different to the latest 5.19.*. Do not use it.
I'm not sure, where are you heading with your question title. You do not want to "call WinSCP without host key"
. The hostkey should be included in your script. See:
I'm not even sure, what you ask for. You seem to have valid URL (or whatever) from your first machine. Just use that.
If you want help with your script, we need to see it. And any error you are getting. Even better a session log file.