I would just see what the minimum fields you can give during domain creation in the UI are, and then take those in via the script. For sure, domain, email, password. I believe all that needs to be done is to insert the record into the admin and domain tables, and then call the request manager script. Not 100% on that though, but it should be a good start.
Some problems though... I believe the welcome email is done through the GUI, where this should be moved into the engine at some point (for reasons like this). So if you want to send the welcome email, you'll have to do that on your own...
Hope this helps...
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pgentoo