[exim] How to configure sieve filtering?

I have the following trivial .forward file:

# Sieve filter
require ["fileinto"];

The presence of this file hods up incoming mail due to:

... save inbox <kaz@???> R=userforward T=address_file defer
(-21): appendfile file or directory name "inbox" is not absolute

I understand that the Sieve filter requires
"inbox" to be mapped to something.

I have the multi-file "conf.d" type configuration, rather
than the all-in-one-file configuration.

I tried adding both "directory = ..." and "file = ..." configuration
lines to /etc/exim4/conf.d/transport/30_exim4-config_address_file,
based on the "inbox" rewriting example in chapter 26 of the
documentation.
Of course, I restarted the server. It made no difference; exim4 keeps
complaining about "inbox" not being absolute.

The "Hello, world" goal here is that messages which are not filtered
are to go into $home/Maildir in the maildir format, as if there was no
.forward file at all.