amazon web services – How to enable HTTPS for my marketplace

I’m building a marketplace platform that enables many merchants to sell to their customers. At the moment I serve stores on my own domain like so https://storeA.mydomain.com or https://storeB.mydomain.com (I’m able to use the subdomain to distinguish which store is to be served). However, I’d like to enable my merchants to use their own domains on my platform whilst being able to secure the site over HTTPS, how can I achieve this automatically? In the past, I’ve tried CNAME to CNAME but this doesn’t allow HTTPS.

EDIT: If you know of any intermediary services (that offer some sort of domain masking) that the merchants would be able to use that would be great!