You cannot use Pageant this way.
See https://winscp.net/eng/docs/faq_scheduler

Hi

I am haviong trouble using windows scheduler to automatically login and upload a file to a webserver configured with SSH

I have a ssh connection from a windows server to a website

I have generated private keys from the website and these are installed via Putty and pageant is acting as the keyfile agent

All works well when the winscp is run manually or in the console via a batch file

winscp calls a script and logs on to the server using sftp

The winscp script file works, the logs show pagaent being used as the agent for the key file

When I schedule the same batch file, or schedule winscp with its relevant switches or arguments, it appears that the sheduled task has issues trying to see pageant ( the logs do not show pageant being referenced when the process is run as a task

It looks to me like the scheduled task is not running in the same session as pageant so cannot see pagaent