How can I setup the custom domain

Here are the steps to set up a custom domain: 

1. You will have to disable the image replacement from the WordPress plugin until the process is finished.
2. You will have to go to https://dashboard.optimole.com/custom_domain and add the custom domain you will want to use instead of your_key.optimole.com ( eg. cdn.example.com 
3. A certificate will be created for that domain, we will provide you with some DNS Records that you need to add inside your DNS for the domain you are trying to configure. This will validate the certificate
4. After the certificate is validated, you will have to modify your wp-config.php so that you specify the usage of the new custom domain
5. You can reactivate the image replacement from the WordPress plugin.

If you encounter any issues at any of the steps mentioned before you can let us know and we will help you resolve any issues.
Take note that a certificate must be validated within 72hours from the time it was generated else it will expire.

If you are using Cloudflare as a DNS provider - 
You will have to allow AWS to validate certificates for you as specified here:

https://support.cloudflare.com/hc/en-us/articles/115000310832-Certification-Authority-Authorization-CAA-FAQ

Just add the following authorities for your domain:
example.com. CAA 0 issue "amazon.com"
example.com. CAA 0 issue "amazontrust.com"
example.com. CAA 0 issue "awstrust.com"
example.com. CAA 0 issue "amazonaws.com"

This should allow AWS to emit an SSL certificate for you and validate it.