A fully qualified domain name (FQDN), sometimes also referred to as an ‘Absolute Domain Name’, the ‘Domain Name’, or ‘Common Name’ is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS).
You must specify the FQDN when filling in the Certificate Signing Request form. For example, if you wish to secure the https://yoursite.com/about.html, the ‘Domain Name’ or ‘Common Name’ is Yoursite.com.
As you can see, the FQDN doesn’t include the protocol name (https://) nor the subpages or subcategories (about.html).
Please note, when requesting a Wildcard SSL certificate, you must add an asterisk before your Domain Name. For instance, *.yourdomain.com.
Source: Sectigo’s Knowledge Base