HTTPS www. problem

I am getting different answers from different people, and it seems like everyone is pointing their finger elsewhere. Here's from a chat with my tech support:
John typically a csr is given for a domain name would need to
add www to the cert at the ssl provider
KGH Omnis tech said, "You would need to add www to the
cert at the CA as we cannot bind a separate cert to www. Once added
please send to the cert back to us and we can do this." I have no
way to do this. ZeroSSL is taking the domain name from the CSR and not
giving me a way to add the www. variation.

Here's my thread at the Let's Encrypt bulletin board about the setup:

I don't have a private "domain" key; my hosting service has that. They give me the CSR, but only for the bare domain. The control panel does not say what software it is, nor the version. They don't even tell me what server OS is running, though I think it is some flavor of Linux.

Yes, I have to email them the certificate every 90 days. Bless Let's Encrypt for affordable SSL!

The big Q is, is there a way to add the www. variant to the bare URL CSR in ZeroSSL?

Thanks very much for your help, KGHN