API Here: http://msdn.microsoft.com/en-us/library/windowsazure/jj157194
In Request Body, I can see how to join an existing domain, but how to create a domain?
Since I need to write code to provision a VM, I wonder whether it is possible to define a new domain forest?
You can’t directly create the VM as AD DC. Moreover, all the tutorials I’ve seen suggest that you use the cloud for Replica AD or Forest AD, and not for primary DC.
Check out these tutorials: