No, there's no default location.
I've documented the syntax:
https://winscp.net/eng/docs/commandline#copyid

Thanks Martin
if I run

I get:

Identity/key file was not specified.

and no logfile created
If I run it with

it works. Is there a default .pub location?
Henry

Command-line interface implemented:
Issue 2342 – Command-line interface for installing public key into server

Martin, Thanks for the response. It's not a big deal. I was curious how it worked.
Henry

The "Install Public Key into Server" uses pure SFTP, so it cannot do any "grep". It downloads and checks locally. I'll consider adding a command-line equivalent.

When I use Tools > Install Public Key into Server command, I notice it checks to see if the key already exists. Do you download authorized_keys and check it locally, or call grep on the remote server? I wanted to add a new key to a list of servers, and did it using put -append, but that doesn't check if the key is already on that server. I didn't see a way to get the result code from a call, other than creating some type of flag.
Thanks,
Henry