How to create a website automatically at registration ?

I installed BuddyPress and Pro Sites in order to sell blogs on my network.
I configured the plugin, everything is OK but that is not exactly what I wanted to do.
In an ideal world on an ideal website, my users shouldn't have the "yes, I want to create a new site" to tick and it would be required to create a website and pay for it at the registration.
On other words : no need to create a website, no need to create an account.
Can you tell me if there a way to do that and lead me the way for how ?
Thanks
Caroline