I just downloaded the free Dorico. There was an initial activation code for set up. But no second activation code was sent after download of software for My Licenses
There is no second activation code, all you need to do is to start the Activation Manager, make sure you are logged in with the correct account and click on the Activate button.
According to YouTube and following the My Activation License there is a longer code
The code you receive via mail needs to be entered in the Download Assistant. There you also log in to your Steinberg account and that will register your trial license at Steinberg.
No idea what Youtube video you have seen, but not everything in Youtube is correct.