Hey there!
First of all I want to thank the whole team at WIX for creating such an amazing software for such a long time.
Being an agency makes us want to white label the domain for all sites we create when we send previews to our customers.
I want to send this:
https://sites.mycompanyname.com/sitename
Instead of this:
https://mycompanyname.wixsite.com/sitename
I am willing to pay for an agency account in some way to make this happen and I believe a lot will do. We used to make sites also in Duda One and they have a great way of whitelabeling this process towards clients.