Thanks again, I appreciate it very much.
I don’t know what keyserver to supply as an argument to apt-key in step 2 of the tutorial.
So I saved the key A0F0E3D9922C071B to a file lock042.gpg and run sudo apt-key add lock042.gpg but got the error gpg: no valid OpenPGP data found. Now I have an empty file /etc/apt/trusted.gpg.d/lock042_ubuntu_siril.gpg.