Unfortunately, Apple Mail does not have an auto BCC feature. Developing one ourselves is in our development plan. In the meantime, there is a third-party paid tool called Mail Butler that allows you to add an auto BCC feature to macOS Mail. They have good setup instructions.
In the absence of an auto BCC feature, add your Clockk BCC
...@secure.clockk.com email address to your Contacts, and try to remember to add it to all your outgoing emails.