At the moment the only way for customers to submit personalised information is via a html text field placed within the item description, with its own separate submit button. There is currently no way of linking that with the add to cart button at this moment in time. Whilst the custom text feature may become available at some stage, the current development focus has been on adding mainstream features to the pro version, based on extensive prior feedback received from existing cart creator users within the forums.
While of course I appreciate that input from users is important, I do think that some research should have been done how how to make a truly professional product, which includes selling more than just static objects. I would suggest that you all look at the process for creating a paypal add-to-cart button, the product options, they are just form fields. You can create one drop down with dollar amounts, and other plain drop downs, and a number of text field inputs. Your own form builder does almost the same thing (no upcharges though of course). Trying to tie user-added code in the details page is the wrong way to go about it, if that's what you are saying. The text input needs to be one of the product options, just add a text field input option in addition to the drop down option types in the Optional Fields area..