[Mailman-Users] Custom key/values in auto-responses

Hi all,

Running my second install of Mailman, going well, v 2.1.11 on BSD 7.0,
and wondering how to add custom key into welcome messages.
Essentially, I need to add a download link into the welcome emailer,
but it needs to have the email of the recipient inserted into it -
it's a one-time expiring download thank you (of my photographs,
actually.)

Seems non-trivial - anyone else ever tried to do this?

Best

Vish, London, UK