I thought you were referring to a certificate for the ISPConfig interface. I know I can request a certificate manually but was hoping I could get ISPConfig to use dns_pdns for newly created websites without needing to use the terminal. acme.sh was installed by the ISPConfig auto-install, so I assumed I wouldn't have to touch that apart from directing it to not use HTTP-01. For the domain for which I issued that exact command, the dns_pdns plugin was indeed used. But not for subsequent websites when I ticked the SSL LE options for each website. It was when I first ran this command I was prompted to register, by that time I didn't yet know that ZeroSSL is acme.sh's default CA.