The documentation states that custom domains can be added by right-clicking on the extension and choosing "Enable CodeCopy on this domain".
https://github.com/zenorocha/codecopy#custom-domains
I have installed the extension on multiple browsers (Chrome and Brave) on multiple OSes (Linux and OSX), and am unable to get this option to show up. Right-clicking on the extension gives the same context menu as left-clicking, and there is no "Enable CodeCopy on this domain".