I'm new to authorize and I'm currently trying to implement DPM on my website. I have read that you can add merchant defined data fields just not sure how.

I want to add the users email so a email receipt can be sent to the user and since I required the users email when they register to my site. I was wanting to retrieve that from my database and send it with the payment form. I'm guessing I can just put a hidden field in the form?