Hi,
An address needs to be associated to a user entry.
So, supposing that you're not logged in, and supposing that you have activated the guest checkout you would only need to have data[register] in order to get the email address of the customer.
If you are in registration mode, then you should also get the username and password with data[register][username] and data[register][password]
If you allow the customer to choose his registration mode (guest or registration), then you would also need data[register][registration_method].
Basically, the same inputs that you would see on the normal HikaShop registration page based on how your checkout is configured.[email] in order to get the email address of the customer.
If you are in registration mode, then you should also get the username and password with data[register][username] and data[register][password]
If you allow the customer to choose his registration mode (guest or registration), then you would also need data[register][registration_method].
Basically, the same inputs that you would see on the normal HikaShop registration page based on how your checkout is configured.