shared mailbox not visible, acl_shared_dict stays empty

I am creating the mailbox like this:

doveadm -o mail_gid=testgroup2 mailbox create -u usertest shared/sharedtest1

I can set acl's (I think), at least the acl get produces the same.

doveadm acl set -u usertest shared/sharedtest1 user=usertest6 lookup read write write-seen insert post doveadm acl get -u usertest shared/sharedtest1

However this file stays empty, I have read somewhere it should automatically be updated?

acl_shared_dict = file:/tmp/shared-mailboxes.db

How should I add entries manually there with:

doveadm dict set file:/tmp/shared-mailboxes.db shared/sharedtest2 kempotest6

or with this syntax still???? https://www.dovecot.org/list/dovecot/2009-April/038922.html