If you want to use gpg-preset-passphrase to store a
passphrase for a OpenPGP key, you need to know the
keygrip of the key. For the primary key this is fairly
easy to get. Either use

  gpg --fingerprint cederp@opera.com

and remove all whitespaces in the fingerprint, or
use something like

  gpg --with-colon --fingerprint cederp@opera.com

and extract the 10th field of the fpr line.

But for subkeys used for encryption (see T1611
for background) this is harder. In fact, I have not
found any way short of running gpg-agent with debug
enabled and look at what gpg sends to gpg-agent.

There should be a way to get the full 20-byte keygrip
of both the main key and all subkeys in a form that is
easy to use with gpg-preset-passphrase. Either the
--with-colon output could be extended, or a new option
such as --keygrip that prints the info in the clear
could be added.