smtpd new "relay as" syntax?

I just upgraded to OpenBSD 6.4, and I'm trying to figure out how to do
this with the new syntax:

accept from local for any relay via smtp://smtp.domain.com as "@domain.com"

This would rewrite the outbound message to masquerade as being from the
TLD rather than a specific machine. Right now I've got:

action local_relay relay host smtp.domain.com
match from local for any action local_relay

But this doesn't do the rewriting. The only thing I see in the man page
talks about 'senders <users> [masquerade]' which seems to be for
authenticated users.

Am I missing something obvious?

Thanks...