This article explains how to add domains to the following multi-domain certificates purchased from SSLs.com:
If your certificate has been already activated, the number of additional domains in your multi-domain certificate can be changed by initiating the reissuance process. If the certificate has not been activated yet, it can be done during activation as well when you need to include more domains in the certificate by default.
NB: Comodo (now Sectigo) Multi-domain certificate contains 3 slots for domains/subdomains by default.
The activation and reissuance of multi-domain certificates are almost the same except that additional domains have to be included additionally if they have not been originally included into the CSR code. The page for adding more domains/subdomains appears after the CSR code is submitted and the domain present in it is confirmed. In order to add them, it is necessary to click on the “+” in the right top corner:
The additional domains are to be separated with commas, or each domain is to be written in a new row. When the list of domains is completed, click the “Add” button:
The domains /subdomains you have added will be listed below the common name field:
When you click the “Onward” button, you will be redirected to a new page where the validation method can be chosen. Please keep in mind that you need to choose either approval email address or HTTP validation for each domain you indicated previously.