preseed repo key

[Javier Marcon <javiermarcon@gmail.com>]

Hello, I can add repositories on a preseed file specifiying the url and
the key file. but how can I make the installer to download the key? For
example I want to add a repository and to get the key file I have to run
the command:

apt-key adv --recv-keys --keyserver keys.gnupg.net E1F958385BFE2B6E

Thanks,

Javier.