Hi,
To run a PowerShell script in a scheduled task, specify powershell.exe in Program/script and -file "C:\path\of\script.ps1" in Add arguments. In the PowerShell script say C:\winscp.ps1 you can add the winscp commands like
winscp.exe "your site"
I don't use WinSCP but you can refer to the WinSCP site for the detailed commandline options.