On our portal, we create a new subdomain for the new paid users ( user1.site.com, user2.site.com and etc.). Working with Kentico 12. As I can see in the documentation, it needs to manually open client.portal, create a new license for a new subdomain and then add this license through the admin dashboard.
Can I automate this process from ASP.NET MVC project using C#?
No, there is no API for this publicly available. You need to contact your account manager using sales@kentico.com to see if there is anything they can do for you.