PUSHR allows you to use your CDN URL with a private SSL certificate issued by the free certificate authority Let's Encrypt. Once issued, the certificate will be applied immediately. PUSHR takes care to renew the certificate for you when it's about to expire.
In order for a SSL certificate to be issued for a branded CDN URL, you have to visit your DNS provider's control panel first and create a CNAME record (eg. cdn.mydomain.com) which resolves to your CDN URL.
Ensure that propagation of the DNS record has taken place before proceeding or the process will fail.
While it's beneficial to have a branded CDN URL, your certificate can work perfectly fine without it. To avoid the need to create a CNAME record, enter your current CDN URL in the pop-up window instead.
Once done, enter the newly created subdomain in the SSL pop-up window and click the Issue certificate button.
PUSHR will initiate the process and your CDN URL will become accessible over https://.