[exim] transport_filter for just a redirect?

I'm trying to add a signature to just one redirect router, but I can't
figure out the syntax. I had it working on all emails with
transport_filter and a shell script. Is there a way to apply that to
just one redirect?

Example redirect:
groupmail_lists:
driver = redirect
local_part_suffix = -group
data = ${lookup mysql {MYSQL_GROUPLOOKUP}}
forbid_file
forbid_pipe
skip_syntax_errors

I'm aware of the mail integrity issues from other posts I've read.