I followed the above example -
bench new-site order1.site
bench --site order1.site install-app erpnext
bench setup add-domain --site order1.site order1.mydomain.com
You can also issue a wildcard SSL certificates which allows all subdomains to be recognized using only one certificate for *.mydomain.com.
To have a wildcard SSL certificate, you must have access to your DNS nameserver because lets-encrypt will give you a code you must place in that DNS nameserver as a cname entry. When lets-encrypt sees that you were able to place that entry to the nameserver, it is confident that you own the domain name, and will issue you a wildcard certificate for the entire domain, mydomain.
Thank you very much @TurkerTunali and @Joseph_Marie_Alba1 for your input and
explanation and for clarifying the wildcard option. Yes i do have access to my DNS
nameserver so that is an option.