When you put your website domain as the Origin-Source, the copy of your website that located on your personal domain name (cname) could be indexed by search engines.
If you want to prevent it:
Create a folder on the origin and add a robots.txt file with the following settings:
Create a Rule for your CDN-resource with the following settings:
Match Type: Regular expression
Rule pattern: robots.*
Rewrite: /(.*) /folder/$1
Where folder is the name of the folder that you’ve created in the first step.
How it is going to work?
The robots.txt file controls how search engine spiders see and interact with your webpage.
The added rule allows us to rewrite a path for robots.txt that will be used by web crawlers. For example, if your personal domain is cdn.domain.com, the search engine crawlers will request the cdn.domain.com/robots.txt URL which contains restrictions for indexing.
Consequently, domain cdn.domain.com won’t be indexed.
! These settings don’t affect the website itself.