Configuring postfix and opendkim socket value
If you want OpenDKIM to open a listening socket on port 8891, put the following in /etc/opendkim.conf.
Socket inet:[email protected]
Postfix will then try to reach this milter using the smtpd_milters setting shown in your question.
Follow one of many, many online guides available for how to set up OpenDKIM. For example the quickstart in Debian wiki.