Use synchronize remote -criteria=none c:\source /dest

Assuming the source is local and the destination is remote, you didn't specify the direction.

See
https://winscp.net/eng/docs/scriptcommand_synchronize
https://winscp.net/eng/docs/ui_synchronize#criteria

I want to copy all the files in my source directory to the destination directory. If a file already exists in the destination, whether it is older or newer, I want to keep it.

Any help much appreciated. Thanks.