To issue an SSL you must prove domain ownership.
Email (easiest)
A verification email is sent to admin@, webmaster@ or the WHOIS contact. Click the link.
HTTP file
A file is placed at /.well-known/pki-validation/HASH.txt in your site root. Auto-checked.
DNS CNAME
A special CNAME record is added to DNS. Works even if your site or mail isn't ready.
Tip
Email is fastest. If you have no mail — use HTTP or DNS.