[exim] Pipe message from filter to return header

OK - here's what I want to do - I want to run a program from the system
filter and return the results in a header - something like this:

headers add "X-Spamprobe: ${run {/etc/exim/scripts/spamprobe-test
$message_id}{$value}{}}"

However - I need to pipe the message into the program that I'm trying to
get the results from. How do I do the piping part?