Howto add ssh keys to .gnupg/sshcontrol?

[Michael Bienia]

Hello,

I'm having trouble to add my ssh key to a running gpg-agent (started
with --enable-ssh-support).

The comments in .gnupg/sshcontrol suggests you can do it with ssh-add or
manually. When I try it with ssh-add I'm asked about my passphrase but
afterwards ssh-add -l doesn't list it and it also doesn't show up in
sshcontrol.
The comment also mentions that one can add it manually by adding a
keygrip of 40 hex digits. How do I get this keygrip from my ssh key to
add it manually?

Thanks,

Michael