This feature is not working in the 3.5.6 version.
I have gone into Edit -> Preferences -> General -> Auto-copy Username and checked it.
Tried it on and off several times and still no-go.
Thank you for contacting us and I'm sorry about the issue you are experiencing. It would seem that the auto-copy option is no longer working in mSecure 3.5.6. At least it is also not working on my Windows 10 (1803) machine. I've tried resetting the config file, and restarting my computer with no luck. I will have to check with others but it appears to simply no longer work. Hopefully, I can find a fix for your issue here. If I cant, unfortunately, the development of mSecure 3 stopped with the release of mSecure 5 so there will be no update of mSecure 3 to fix this issue.