EHCP Force can seamlessly configure all of this for you. Simply go to advanced settings in the panel and pick the master domain you want to use:

Now to view or retrieve the actual DNS record (if needed for something else or a more advanced operation), go to the DKIM domain in the control panel, and click on "Custom DNS". You'll see a similar record to:

This functionality hooks into the custom DNS feature for the selected domain and updates everything needed to make it work automatically.
All domains configured in the panel will use that DKIM record for emails.