You can add direct references to your database in the server side code. There is no way to do it entirely in the html. You should be able to open up the add to cart server behavior and set the values to come from an existing database using the lightning bolts.
The cart doesn't look for defaults, it just creates form elements using a particular syntax if you use it to create the form elements. It can be integrated with a custom form or database, but that is done in the php code and not in the html alone. That is why you should be using the server behavior to make these edits.
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