I need to add an extra domain to my existing certificate. I’ve read a few threads dealing with this question, but I’m being told that the way to do this will differ depending upon the command I used to create the initial certificate.
You can run the same command again. The important thing to know is that you have to type all of the names that should be on the new certificate—not just the new name! If you omit some or all of the existing names, you’ll get a separate somewhat duplicative certificate that doesn’t replace the old one.
Certbot should then prompt you with a question about whether you want to expand the existing certificate, and you can tell it that yes, you do.
If you want a reminder about exactly what names are on the old certificate, you can run ./certbot-auto certificates.