[exim] system_filter_pipe_transport

I am attempting to encrypt messages by using a filter to pass messages
to a python script. The content of exim filter is:

# Exim filter
pipe /root/encryption/encrypt.py

When I try to test this I see:

LOG: MAIN PANIC
== |/root/encryption/encrypt.py <system-filter> routing defer (-1):
system_filter_pipe_transport is unset

This page:

https://www.exim.org/exim-html-current/doc/html/spec_html/ch-systemwide_message_filtering.html

says:

If a system filter generates any deliveries directly to files or pipes
(via the save or pipe commands), transports to handle these deliveries
must be specified by setting system_filter_file_transport and
system_filter_pipe_transport, respectively.

but where do I set them and what do I set them to? An example would be
helpful.

Thanks!