Thank you so much, this is seems solution for my issue now.
but I found this link - How many domains in a certificate? - #3 by jmorahan - seems I can only add up to 100 domain names per cert?
This rate is super too low for a site builder with thousands of created sites.
I also found that Wix shall issue an own cert for each domain, Wix works as registrar & hosting provider like BlueHost?
If my builder runs as registrar & hosting provider, there's any solution for me to issue own cert for each sold domain automatically?
Or if my users have a domain at some registrar, they would buy/get a some SSL cert (such as sslforfree.com), how to assign this cert to my builder to accept user's domain via SSL?
Thanks