Adding a subdomain pointing to an external service

I want to point a subdomain to an external service which is hosted on AWS EC2. I would like to enable SSL on my EC2 for which I need a certificate. How do I work this? I dont mind paying for the certificate and install it on my EC2. But I am lost how i can tie the certificate to my domain which is hosted by Wix.

Lets say I have www_mycompany_com hosted on Wix. I have a separate service blog_mycompany_com which I am running on EC2. How can I use a certificate tied to blog_mycompany_com that is installed on EC2?