You should enter the exact domain name or subdomain you wish to secure in the Common Name (CN) field. For example: if you intend to secure the URL https://www.yourdomain.com, then your CSR's Common Name must be www.yourdomain.com. Please do not include http:// or https:// in your CSR's Common Name.
Will the certificate work for domain.com or www.domain.com?
For your convenience, we created an article explaining how different certificates we provide work with these kinds of common names. And here's an article about the common name field peculiarities for the good CSR.