Insert folder name for FCC header field

When I compose mail, I often want to save a copy of the outgoing message in
a specific folder.  The header field FCC can be used to specify a folder,
its default value stored in the wl-fcc variable.

However, currently the only way to edit the FCC field (as far as I can tell)
is by hand.  It would be great if a folder name could be read from the
minibuffer (with autocompletion) that would replace the FCC field.  So when
you wanted to save a copy of the outgoing message, you could invoke the
command, select the folder you want, and a copy would be saved there instead
of or in addition to the default.

Curiously, the documentation of wl-draft-fcc says "Add a new Fcc field, with
file name completion."  But I can't figure out how to trigger the file name
completion, and all the function seems to be doing is to insert the Fcc:
field.

Does anyone have any advice how to activate the autocompletion or tell me
whether it's possible?

Thanks,
Greg