Hi @CATKINS,
In this case it appears that your hosting provider needs to help you with this process. Adobe Business Catalyst does not appear to offer the kind of access that would allow you to do this for yourself.
I read conflicting things about the availability of support for HTTPS on a custom domain on this hosting provider, but they made an announcement earlier that it would be available during 2017:
https://www.businesscatalyst.com/bc-blog/welcome-to-the-new-face-of-business-catalyst
So you should ask the hosting provider's support about the status of this. Although you can obtain a third-party certificate (such as a Let's Encrypt certificate) for your site without the hosting provider's help, you won't be able to install it without the hosting provider's permission and/or assistance, so it wouldn't do you any good.
If Adobe Business Catalyst isn't able to allow you to use HTTPS on a custom domain, you could consider any of the hosting providers listed in the thread at
By the way, a CSR is a "certificate signing request" which is a kind of document used in some workflows as part of the process of obtaining a certificate. You won't necessarily need to use a CSR directly; what you're probably looking for is the certificate. 